OAuth Working Group D. Hardt Internet-Draft SignIn.Org Intended status: Standards Track A. Parecki Expires:16 September 202112 March 2022 Okta T. Lodderstedt yes.com15 March8 September 2021 The OAuth 2.1 Authorization Frameworkdraft-ietf-oauth-v2-1-02draft-ietf-oauth-v2-1-03 Abstract The OAuth 2.1 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and an authorization service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 2.0 Authorization Framework described in RFC 6749. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on16 September 2021.12 March 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . 7 1.3. Authorization Grant . . . . . . . . . . . . . . . . . . . 8 1.3.1. Authorization Code . . . . . . . . . . . . . . . . . 8 1.3.2.Client Credentials . .Refresh Token . . . . . . . . . . . . . . .9 1.4. Access Token. . . . . 9 1.3.3. Client Credentials . . . . . . . . . . . . . . . . .9 1.5. Refresh10 1.4. Access Token . . . . . . . . . . . . . . . . . . . . . .10 1.6.11 1.5. TLS Version . . . . . . . . . . . . . . . . . . . . . . . 121.7.1.6. HTTP Redirections . . . . . . . . . . . . . . . . . . . . 121.8.1.7. Interoperability . . . . . . . . . . . . . . . . . . . . 121.9.1.8. Compatibility with OAuth 2.0 . . . . . . . . . . . . . . 131.10.1.9. Notational Conventions . . . . . . . . . . . . . . . . . 13 2. Client Registration . . . . . . . . . . . . . . . . . . . . . 14 2.1. Client Types . . . . . . . . . . . . . . . . . . . . . . 14 2.2. Client Identifier . . . . . . . . . . . . . . . . . . . . 16 2.3. ClientAuthentication . . .Redirection Endpoint . . . . . . . . . . . . . . . 16 2.3.1.Client SecretEndpoint Request Confidentiality . . . . . . . . . . 16 2.3.2. Registration Requirements . . . . . . . . . . .17 2.3.2. Other Authentication Methods. . . 17 2.3.3. Multiple Redirect URIs . . . . . . . . .18 2.4. Unregistered Clients. . . . . . 17 2.3.4. Invalid Endpoint . . . . . . . . . . . .18 3. Protocol Endpoints. . . . . . 17 2.3.5. Endpoint Content . . . . . . . . . . . . . . .18 3.1. Authorization Endpoint. . . 17 2.4. Client Authentication . . . . . . . . . . . . . .19 3.1.1. Response Type. . . . 18 2.4.1. Client Secret . . . . . . . . . . . . . . . .19 3.1.2. Redirection Endpoint. . . . 19 2.4.2. Other Authentication Methods . . . . . . . . . . . . 203.2. Token Endpoint2.5. Unregistered Clients . . . . . . . . . . . . . . . . . . 20 3. Protocol Endpoints . . .22 3.2.1. Client Authentication. . . . . . . . . . . . . . . .22 3.3. Access Token Scope. . 20 3.1. Authorization Endpoint . . . . . . . . . . . . . . . . .23 4. Obtaining Authorization21 3.2. Token Endpoint . . . . . . . . . . . . . . . . . . .23 4.1. Authorization Code Grant. . 21 3.2.1. Client Authentication . . . . . . . . . . . . . .23 4.1.1. Authorization. . 22 3.2.2. Token Request . . . . . . . . . . . . . . . .25 4.1.2. Authorization Response. . . . 22 3.2.3. Token Response . . . . . . . . . . .28 4.1.3. Access Token Request. . . . . . . . 24 4. Grant Types . . . . . . . .31 4.1.4. Access Token Response. . . . . . . . . . . . . . . .32 4.2. Client Credentials. 27 4.1. Authorization Code Grant . . . . . . . . . . . . . . . .32 4.2.1.28 4.1.1. Authorization Requestand Response. . . . . . . . .33 4.2.2. Access Token Request. . . . . . . 29 4.1.2. Authorization Response . . . . . . . . .33 4.2.3. Access Token Response. . . . . . 32 4.1.3. Token Endpoint Extension . . . . . . . . . .34 4.3. Extension Grants. . . . 35 4.2. Client Credentials Grant . . . . . . . . . . . . . . . .34 5. Issuing an Access36 4.2.1. Token Endpoint Extension . . . . . . . . . . . . . . 37 4.3. Refresh Token Grant . . . . .35 5.1. Successful Response. . . . . . . . . . . . . . 38 4.3.1. Token Endpoint Extension . . . . .35 5.2. Error Response. . . . . . . . . 38 4.3.2. Refresh Token Response . . . . . . . . . . . .36 6. Refreshing an Access Token. . . 39 4.4. Extension Grants . . . . . . . . . . . . . .38 6.1. Refresh Token Request. . . . . . 40 5. Accessing Protected Resources . . . . . . . . . . . .38 6.2. Refresh Token Response. . . .. . . . . . . . . . . . . 40 7. Accessing Protected Resources . . . . . . . . . . . . . . . . 41 7.1. Access Token Types41 5.1. Access Token Types . . . . . . . . . . . . . . . . . . . 417.2.5.2. Bearer Tokens . . . . . . . . . . . . . . . . . . . . . . 427.2.1.5.2.1. Authenticated Requests . . . . . . . . . . . . . . . 427.2.2.5.2.2. The WWW-Authenticate Response Header Field . . . . . 447.2.3.5.2.3. Error Codes . . . . . . . . . . . . . . . . . . . . . 457.3.5.3. Error Response . . . . . . . . . . . . . . . . . . . . . 467.3.1.5.3.1. Extension Token Types . . . . . . . . . . . . . . . . 467.4. Access Token Security Considerations6. Extensibility . . . . . . . . . .46 7.4.1. Security Threats. . . . . . . . . . . . . . 46 6.1. Defining Access Token Types . . . .47 7.4.2. Threat Mitigation. . . . . . . . . . . 47 6.2. Defining New Endpoint Parameters . . . . . . .47 7.4.3. Summary of Recommendations. . . . . 47 6.3. Defining New Authorization Grant Types . . . . . . . .49 7.4.4. Token Replay Prevention. 47 6.4. Defining New Authorization Endpoint Response Types . . . 48 6.5. Defining Additional Error Codes . . . . . . . . . . .50 7.4.5. Access Token Privilege Restriction. . 48 7. Security Considerations . . . . . . .51 8. Extensibility. . . . . . . . . . . . 49 7.1. Access Token Security Considerations . . . . . . . . . . 49 7.1.1. Security Threats . .51 8.1. Defining Access Token Types. . . . . . . . . . . . . . .51 8.2. Defining New Endpoint Parameters. 49 7.1.2. Threat Mitigation . . . . . . . . . . .52 8.3. Defining New Authorization Grant Types. . . . . . . 50 7.1.3. Summary of Recommendations . .52 8.4. Defining New Authorization Endpoint Response Types. . .52 8.5. Defining Additional Error Codes. . . . . . . . 52 7.1.4. Token Replay Prevention . . . . .53 9. Security Considerations. . . . . . . . . . 53 7.1.5. Access Token Privilege Restriction . . . . . . . . .53 9.1.54 7.2. Client Authentication . . . . . . . . . . . . . . . . . . 549.1.1.7.2.1. Client Authentication of Native Apps . . . . . . . .54 9.2.55 7.3. Registration of Native App Clients . . . . . . . . . . . 559.3.7.4. Client Impersonation . . . . . . . . . . . . . . . . . .55 9.3.1.56 7.4.1. Impersonation of Native Apps . . . . . . . . . . . . 569.4. Access Tokens . . . . . . . . . . . . . . . . . . . . . . 56 9.4.1.7.4.2. Access Token Privilege Restriction . . . . . . . . . 579.4.2.7.4.3. Access Token Replay Prevention . . . . . . . . . . . 579.5.7.5. Refresh Tokens . . . . . . . . . . . . . . . . . . . . . 589.6.7.6. Client Impersonating Resource Owner . . . . . . . . . . . 589.7.7.7. ProtectingRedirect-Based Flows . . . .the Authorization Code Flow . . . . . . . . . 599.7.1.7.7.1. Loopback Redirect Considerations in Native Apps . . . 599.7.2.7.7.2. HTTP 307 Redirect . . . . . . . . . . . . . . . . . . 609.8.7.8. Authorization Codes . . . . . . . . . . . . . . . . . . . 619.9.7.9. Request Confidentiality . . . . . . . . . . . . . . . . . 629.10.7.10. Ensuring Endpoint Authenticity . . . . . . . . . . . . . 629.11.7.11. Credentials-Guessing Attacks . . . . . . . . . . . . . .63 9.12.62 7.12. Phishing Attacks . . . . . . . . . . . . . . . . . . . . 639.13.7.13. Fake External User-Agents in Native Apps . . . . . . . . 639.14.7.14. Malicious External User-Agents in Native Apps . . . . . . 649.15.7.15. Cross-Site Request Forgery . . . . . . . . . . . . . . . 649.16.7.16. Clickjacking . . . . . . . . . . . . . . . . . . . . . . 659.17.7.17. Code Injection and Input Validation . . . . . . . . . . . 669.18.7.18. Open Redirectors . . . . . . . . . . . . . . . . . . . . 669.18.1.7.18.1. Client as Open Redirector . . . . . . . . . . . . . 669.18.2.7.18.2. Authorization Server as Open Redirector . . . . . . 669.19.7.19. Authorization Server Mix-Up Mitigation in Native Apps . . 679.20.7.20. Embedded User Agents in Native Apps . . . . . . . . . . . 679.21.7.21. Other Recommendations . . . . . . . . . . . . . . . . . . 6810.8. Native Applications . . . . . . . . . . . . . . . . . . . . . 6810.1.8.1. Using Inter-App URI Communication for OAuth in Native Apps . . . . . . . . . . . . . . . . . . . . . . . . . . 6910.2.8.2. Initiating the Authorization Request from a Native App .. . . . . . . . . . . . . . . . . . . . . . . . .7010.3.8.3. Receiving the Authorization Response in a Native App . .71 10.3.1.70 8.3.1. Private-Use URI Scheme Redirection . . . . . . . . . 7110.3.2.8.3.2. Claimed "https" Scheme URI Redirection . . . . . . . 7210.3.3.8.3.3. Loopback Interface Redirection . . . . . . . . . . . 7211.9. Browser-Based Apps . . . . . . . . . . . . . . . . . . . . . 7312.10. Differences from OAuth 2.0 . . . . . . . . . . . . . . . . . 7313.11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7414.12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7414.1.12.1. Normative References . . . . . . . . . . . . . . . . . . 7414.2.12.2. Informative References . . . . . . . . . . . . . . . . . 77 Appendix A. Augmented Backus-Naur Form (ABNF) Syntax . . . . . . 80 A.1. "client_id" Syntax . . . . . . . . . . . . . . . . . . .8180 A.2. "client_secret" Syntax . . . . . . . . . . . . . . . . .8180 A.3. "response_type" Syntax . . . . . . . . . . . . . . . . .8180 A.4. "scope" Syntax . . . . . . . . . . . . . . . . . . . . . 81 A.5. "state" Syntax . . . . . . . . . . . . . . . . . . . . . 81 A.6. "redirect_uri" Syntax . . . . . . . . . . . . . . . . . . 81 A.7. "error" Syntax . . . . . . . . . . . . . . . . . . . . . 81 A.8. "error_description" Syntax . . . . . . . . . . . . . . .8281 A.9. "error_uri" Syntax . . . . . . . . . . . . . . . . . . .8281 A.10. "grant_type" Syntax . . . . . . . . . . . . . . . . . . .8281 A.11. "code" Syntax . . . . . . . . . . . . . . . . . . . . . . 82 A.12. "access_token" Syntax . . . . . . . . . . . . . . . . . . 82 A.13. "token_type" Syntax . . . . . . . . . . . . . . . . . . . 82 A.14. "expires_in" Syntax . . . . . . . . . . . . . . . . . . . 82 A.15. "refresh_token" Syntax . . . . . . . . . . . . . . . . .8382 A.16. Endpoint Parameter Syntax . . . . . . . . . . . . . . . .8382 A.17. "code_verifier" Syntax . . . . . . . . . . . . . . . . . 83 A.18. "code_challenge" Syntax . . . . . . . . . . . . . . . . . 83 Appendix B. Use of application/x-www-form-urlencoded Media Type . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Appendix C. Extensions . . . . . . . . . . . . . . . . . . . . . 84 Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . .8685 Appendix E. Document History . . . . . . . . . . . . . . . . . . 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 86 1. Introduction In the traditional client-server authentication model, the client requests an access-restricted resource (protected resource) on the server by authenticating with the server using the resource owner's credentials. In order to provide third-party applications access to restricted resources, the resource owner shares its credentials with the third party. This creates several problems and limitations: * Third-party applications are required to store the resource owner's credentials for future use, typically a password in clear- text. * Servers are required to support password authentication, despite the security weaknesses inherent in passwords. * Third-party applications gain overly broad access to the resource owner's protected resources, leaving resource owners without any ability to restrict duration or access to a limited subset of resources. * Resource owners often reuse passwords with other unrelated services, despite best security practices. This password reuse means a vulnerability or exposure in one service may have security implications in completely unrelated services. * Resource owners cannot revoke access to an individual third party without revoking access to all third parties, and must do so by changingthe third party'stheir password. * Compromise of any third-party application results in compromise of the end-user's password and all of the data protected by that password. OAuth addresses these issues by introducing an authorization layer and separating the role of the client from that of the resource owner. In OAuth, the client requests access to resources controlled by the resource owner and hosted by the resource server. Instead of using the resource owner's credentials to access protected resources, the client obtains an access token - a credential representing a specific set of access attributes such as scope and lifetime. Access tokens are issued to clients by an authorization server with the approval of the resource owner. The client uses the access token to access the protected resources hosted by the resource server. For example, an end-user (resource owner) can grant a printing service (client) access to their protected photos stored at a photo- sharing service (resource server), without sharing their username and password with the printing service. Instead, theyauthenticatesauthenticate directly with a server trusted by the photo-sharing service (authorization server), which issues the printing service delegation- specific credentials (access token). This specification is designed for use with HTTP([RFC7230]).([RFC7231]). The use of OAuth over any protocol other than HTTP is out of scope. Since the publication of the OAuth 2.0 Authorization Framework ([RFC6749]) in October 2012, it has been updated by OAuth 2.0 for Native Apps ([RFC8252]), OAuth Security Best Current Practice ([I-D.ietf-oauth-security-topics]), and OAuth 2.0 for Browser-Based Apps ([I-D.ietf-oauth-browser-based-apps]). The OAuth 2.0 Authorization Framework: Bearer Token Usage ([RFC6750]) has also been updated with ([I-D.ietf-oauth-security-topics]). This Standards Track specification consolidates the information in all of these documents and removes features that have been found to be insecure in [I-D.ietf-oauth-security-topics]. 1.1. Roles OAuth defines four roles: "resource owner": An entity capable of granting access to a protected resource. When the resource owner is a person, it is referred to as an end-user. This is sometimes abbreviated as "RO". "resource server": The server hosting the protected resources, capable of accepting and responding to protected resource requests using access tokens. The resource server is often accessible via an API. This is sometimes abbreviated as "RS". "client": An application making protected resource requests on behalf of the resource owner and with its authorization. The term "client" does not imply any particular implementation characteristics (e.g., whether the application executes on a server, a desktop, or other devices). "authorization server": The server issuing access tokens to the client after successfully authenticating the resource owner and obtaining authorization. This is sometimes abbreviated as "AS". The interaction between the authorization server and resource server is beyond the scope of this specification, however severalextensionextensions have been defined to provide an option for interoperability between resource servers and authorization servers. The authorization server may be the same server as the resource server or a separate entity. A single authorization server may issue access tokens accepted by multiple resource servers. 1.2. Protocol Flow +--------+ +---------------+ | |--(1)- Authorization Request ->| Resource | | | | Owner | | |<-(2)-- Authorization Grant ---| | | | +---------------+ | | | | +---------------+ | |--(3)-- Authorization Grant -->| Authorization | | Client | | Server | | |<-(4)----- Access Token -------| | | | +---------------+ | | | | +---------------+ | |--(5)----- Access Token ------>| Resource | | | | Server | | |<-(6)--- Protected Resource ---| | +--------+ +---------------+ Figure 1: Abstract Protocol Flow The abstract OAuth 2.1 flow illustrated in Figure 1 describes the interaction between the four roles and includes the following steps: 1. The client requests authorization from the resource owner. The authorization request can be made directly to the resource owner (as shown), or preferably indirectly via the authorization server as an intermediary. 2. The client receives an authorization grant, which is a credential representing the resource owner's authorization, expressed using one oftwothe authorization grant types defined in this specification or using an extension grant type. The authorization grant type depends on the method used by the client to request authorization and the types supported by the authorization server. 3. The client requests an access token by authenticating with the authorization server and presenting the authorization grant. 4. The authorization server authenticates the client and validates the authorization grant, and if valid, issues an access token. 5. The client requests the protected resource from the resource server and authenticates by presenting the access token. 6. The resource server validates the access token, and if valid, serves the request. The preferred method for the client to obtain an authorization grant from the resource owner (depicted in steps (1) and (2)) is to use the authorization server as an intermediary, which is illustrated in Figure 3 in Section 4.1. 1.3. Authorization Grant An authorization grant is a credential representing the resource owner's authorization (to access its protected resources) used by the client to obtain an access token. This specification definestwothree grant types - authorizationcodecode, refresh token, and client credentials - as well as an extensibility mechanism for defining additional types. 1.3.1. Authorization Code An authorization code is a temporary credential used to obtain an access token. Instead of the client requesting authorization directly from the resource owner, the client directs the resource owner to an authorization server (via itsuser-agent as defined in [RFC7231]),user agent, which in turn directs the resource owner back to the client with the authorization code. The client can then exchange the authorization code for an access token. Before directing the resource owner back to the client with the authorization code, the authorization server authenticates the resource owner, and may request the resource owner's consent or otherwise inform them of the client's request. Because the resource owner only authenticates with the authorization server, the resource owner's credentials are never shared with the client, and the client does not need to have knowledge of any additional authentication steps such as multi-factor authentication or delegated accounts. The authorization code provides a few important security benefits, such as the ability to authenticate the client, as well as the transmission of the access token directly to the client without passing it through the resource owner'suser-agentuser agent and potentially exposing it to others, including the resource owner. 1.3.2.Client Credentials The clientRefresh Token Refresh tokens are credentialsor other forms of client authentication (e.g. a "client_secret" or a private keyused tosign a JWT) can be used as an authorization grant when the authorization scope is limitedobtain access tokens. Refresh tokens are issued to theprotected resources under the control of the client, or to protected resources previously arranged withclient by the authorizationserver. Client credentialsserver and are usedas an authorization grant typicallyto obtain a new access token when theclient is acting on its own behalf (the client is alsocurrent access token becomes invalid or expires, or to obtain additional access tokens with identical or narrower scope (access tokens may have a shorter lifetime and fewer permissions than authorized by the resourceowner) orowner). Issuing a refresh token isrequesting access to protected resourcesoptional at the discretion of the authorization server, and may be issued based onanproperties of the client, properties of the request, policies within the authorizationpreviously arranged withserver, or any other criteria. If the authorizationserver. 1.4. Access Token Access tokens are credentials used to access protected resources. Anserver issues a refresh token, it is included when issuing an access token (i.e., step (2) in Figure 2). A refresh token is a string representinganthe authorizationissuedgranted to theclient.client by the resource owner. The string is considered opaque to theclient, even if it has a structure. Depending on the authorization server, the access token string may be parseable by the resource server. Access tokens represent specific scopes and durations of access, granted by the resource owner, and enforced by the resource server and authorization server.client. The refresh token may be an identifier usedby the RSto retrieve the authorizationinformation,information orthe tokenmayself-contain the authorizationencode this informationin a verifiable manner (i.e., a token string consisting of a signed data payload). One example of a token retrieval mechanism is Token Introspection [RFC7662], in which the RS calls an endpoint on the AS to validate the token presented byinto theclient. One example of a structured token format is [I-D.ietf-oauth-access-token-jwt], a method of encodingstring itself. Unlike accesstoken data as a JSON Web Token [RFC7519]. Additional authentication credentials, whichtokens, refresh tokens arebeyond the scope of this specification, may be required in orderintended forthe client tousean access token. This is typically referred to as a sender-constrained access token, such as Mutual TLS Access Tokens [RFC8705]. The access token provides an abstraction layer, replacing different authorization constructs (e.g., username and password)only witha single token understood by the resource server. This abstraction enables issuing access tokens more restrictive than theauthorizationgrant usedservers and are never sent toobtain them, as well as removing theresourceserver's need to understand a wide range of authentication methods. Access tokens can have different formats, structures, and methods of utilization (e.g., cryptographic properties) based on the resource server security requirements. Access token attributes and the methods used to access protected resources may be extended beyond what is described in this specification. 1.5. Refresh Token Refresh tokens are credentials used to obtain access tokens. Refresh tokens are issued to the client by the authorization server and are used to obtain a new access token when the current access token becomes invalid or expires, or to obtain additional access tokens with identical or narrower scope (access tokens may have a shorter lifetime and fewer permissions than authorized by the resource owner). Issuing a refresh token is optional at the discretion of the authorization server, and may be issued based on properties of the client, properties of the request, policies within the authorization server, or any other criteria. If the authorization server issues a refresh token, it is included when issuing an access token (i.e., step (2) in Figure 2). A refresh token is a string representing the authorization granted to the client by the resource owner. The string is considered opaque to the client. The refresh token may be an identifier used to retrieve the authorization information or may encode this information into the string itself. Unlike access tokens, refresh tokens are intended for use only with authorization servers and are never sent to resource servers. +--------+ +---------------+ | |--(1)------- Authorization Grant --------->| | | | | | | |<-(2)-----------servers. +--------+ +---------------+ | |--(1)------- Authorization Grant --------->| | | | | | | |<-(2)----------- Access Token -------------| | | | & Refresh Token | | | | | | | | +----------+ | | | |--(3)---- Access Token ---->| | | | | | | | | | | |<-(4)- Protected Resource --| Resource | | Authorization | | Client | | Server | | Server | | |--(5)---- Access Token ---->| | | | | | | | | | | |<-(6)- Invalid Token Error -| | | | | | +----------+ | | | | | | | |--(7)----------- Refresh Token ----------->| | | | | | | |<-(8)----------- Access Token -------------| | +--------+ & Optional Refresh Token +---------------+ Figure 2: Refreshing an Expired Access Token The flow illustrated in Figure 2 includes the following steps: 1. The client requests an access token by authenticating with the authorization server and presenting an authorization grant. 2. The authorization server authenticates the client and validates the authorization grant, and if valid, issues an access token and optionally a refresh token. 3. The client makes a protected resource request to the resource server by presenting the access token. 4. The resource server validates the access token, and if valid, serves the request. 5. Steps (3) and (4) repeat until the access token expires. If the client knows the access token expired, it skips to step (7); otherwise, it makes another protected resource request. 6. Since the access token is invalid, the resource server returns an invalid token error. 7. The client requests a new access token by presenting the refresh token and providing client authentication if it has been issued credentials. The client authentication requirements are based on the client type and on the authorization server policies. 8. The authorization server authenticates the client and validates the refresh token, and if valid, issues a new access token (and, optionally, a new refresh token).1.6. TLS Version Whenever Transport Layer Security (TLS) is used by this specification, the appropriate version (or versions) of TLS will vary over time, based on the widespread deployment and known security vulnerabilities. Refer to [BCP195] for up to date recommendations on transport layer security. Implementations MAY also support additional transport-layer security mechanisms that meet their security requirements. 1.7. HTTP Redirections This specification makes extensive use1.3.3. Client Credentials The client credentials or other forms ofHTTP redirections, in which theclient authentication (e.g. a "client_secret" orthea private key used to sign a JWT) can be used as an authorizationserver directsgrant when theresource owner's user-agentauthorization scope is limited toanother destination. Whiletheexamples in this specification showprotected resources under theusecontrol of theHTTP 302 status code, any other method available via the user-agentclient, or toaccomplish this redirection,protected resources previously arranged with theexception of HTTP 307,authorization server. Client credentials are used as an authorization grant typically when the client isallowed andacting on its own behalf (the client isconsideredalso the resource owner) or is requesting access tobeprotected resources based on animplementation detail. See Section 9.7.2 for details. 1.8. Interoperability OAuth 2.1 provides a richauthorizationframeworkpreviously arranged withwell-defined security properties. This specification leaves a few required components partially or fully undefined (e.g., client registration,the authorizationserver capabilities, endpoint discovery). Some of these behaviorsserver. 1.4. Access Token Access tokens aredefined in optional extensions which implementations can choosecredentials used touse, such as: * [RFC8414]: Authorization Server Metadata, definingaccess protected resources. An access token is a string representing anendpoint clients can useauthorization issued tolook uptheinformation neededclient. The string is considered opaque tointeract with a particular OAuth server * [RFC7591]: Dynamic Client Registration, providingthe client, even if it has amechanism for programmatically registering clients with anstructure. Depending on the authorizationserver * [RFC7592]: Dynamic Client Management, providing a mechanism for updating dynamically registered client information * [RFC7662]:server, the access token string may be parseable by the resource server, such as when using the JSON Web TokenIntrospection, defining a mechanismProfile for Access Tokens ([I-D.ietf-oauth-access-token-jwt]). Access tokens represent specific scopes and durations of access, granted by the resourceserversowner, and enforced by the resource server and authorization server. The token may be used by the RS toobtainretrieve the authorization information, or the token may self-contain the authorization informationabout access tokens Please refer to Appendix C forin alistverifiable manner (i.e., a token string consisting ofcurrent known extensions at the timea signed data payload). One example ofthis publication. 1.9. Compatibility with OAuth 2.0 OAuth 2.1a token retrieval mechanism iscompatible with OAuth 2.0 with the extensions and restrictions from known best current practices applied. Specifically, features not specifiedToken Introspection [RFC7662], inOAuth 2.0 core, suchwhich the RS calls an endpoint on the AS to validate the token presented by the client. One example of a structured token format is [I-D.ietf-oauth-access-token-jwt], a method of encoding access token data asPKCE,a JSON Web Token [RFC7519]. Additional authentication credentials, which are beyond the scope of this specification, may be required inOAuth 2.1. Additionally, some features available in OAuth 2.0, such asorder for theImplicit or Resource Owner Credentials grant types, are not specified in OAuth 2.1. Furthermore, some behaviors allowed in OAuth 2.0 are restricted in OAuth 2.1,client to use an access token. This is typically referred to as a sender-constrained access token, such asthe strict string matching of redirect URIs requiredMutual TLS Access Tokens [RFC8705]. The access token provides an abstraction layer, replacing different authorization constructs (e.g., username and password) with a single token understood byOAuth 2.1. See Section 12 forthe resource server. This abstraction enables issuing access tokens moredetailsrestrictive than the authorization grant used to obtain them, as well as removing the resource server's need to understand a wide range of authentication methods. Access tokens can have different formats, structures, and methods of utilization (e.g., cryptographic properties) based on thedifferences from OAuth 2.0. 1.10. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY",resource server security requirements. Access token attributes and"OPTIONAL" in this document arethe methods used to access protected resources may beinterpreted asextended beyond what is described inBCP 14 [RFC2119] [RFC8174] when,this specification. Access tokens (as well as any confidential access token attributes) MUST be kept confidential in transit and storage, and onlywhen, they appear in all capitals, as shown here. This specification usesshared among theAugmented Backus-Naur Form (ABNF) notation of [RFC5234]. Additionally,authorization server, therule URI-referenceresource servers the access token isincluded from "Uniform Resource Identifier (URI): Generic Syntax" [RFC3986]. Certain security-related terms arevalid for, and the client tobe understood inwhom thesense defined in [RFC4949]. These terms include, but are not limited to, "attack", "authentication", "authorization", "certificate", "confidentiality", "credential", "encryption", "identity", "sign", "signature", "trust", "validate", and "verify". The term "payload"access token istoissued. Access token credentials MUST only beinterpretedtransmitted using TLS as described in Section3.31.5 with server authentication as defined by [RFC2818]. The authorization server MUST ensure that access tokens cannot be generated, modified, or guessed to produce valid access tokens by unauthorized parties. 1.5. TLS Version Whenever Transport Layer Security (TLS) is used by this specification, the appropriate version (or versions) of[RFC7231]. Unless otherwise noted, allTLS will vary over time, based on theprotocol parameter nameswidespread deployment andvalues are case sensitive. 2. Client Registration Before initiating the protocol, the client must establish its registration with the authorization server. The means throughknown security vulnerabilities. Refer to [BCP195] for up to date recommendations on transport layer security. Implementations MAY also support additional transport-layer security mechanisms that meet their security requirements. 1.6. HTTP Redirections This specification makes extensive use of HTTP redirections, in which the clientregisters withor the authorization serverare beyonddirects thescope ofresource owner's user agent to another destination. While the examples in this specificationbut typically involveshow theclient developer manually registeringuse of theclient atHTTP 302 status code, any other method available via theauthorization server's website after creating an account and agreeinguser agent to accomplish this redirection, with theservice's Termsexception ofService, or by using Dynamic Client Registration ([RFC7591]). Client registration does not require a direct interaction between the clientHTTP 307, is allowed andthe authorization server. When supported by the authorization server, registration can rely on other meansis considered to be an implementation detail. See Section 7.7.2 forestablishing trust and obtaining thedetails. 1.7. Interoperability OAuth 2.1 provides a rich authorization framework with well-defined security properties. This specification leaves a few requiredclient propertiescomponents partially or fully undefined (e.g.,redirect URI,clienttype). For example, registration can be accomplished using a self-issued or third-party-issued assertion, or by theregistration, authorization serverperforming client discovery using a trusted channel. When registering a client, the client developer SHALL: * specify the client type as described in Section 2.1, * provide client details needed by the grant typecapabilities, endpoint discovery). Some of these behaviors are defined in optional extensions which implementations can choose to use, suchas redirect URIs as described in Section 3.1.2, andas: *include any other information required by[RFC8414]: Authorization Server Metadata, defining an endpoint clients can use to look up theauthorizationinformation needed to interact with a particular OAuth server(e.g., application name, website, description, logo image, the acceptance of legal terms).* [RFC7591]: Dynamic ClientRegistration ([RFC7591]) definesRegistration, providing acommon general data modelmechanism for programmatically registering clientsthat may be used evenwithmanual client registration. 2.1.an authorization server * [RFC7592]: Dynamic ClientTypes OAuth 2.1 defines threeManagement, providing a mechanism for updating dynamically registered clienttypes based on their abilityinformation * [RFC7662]: Token Introspection, defining a mechanism for resource servers toauthenticate securely with the authorization server as well as the authorization server's assuranceobtain information about access tokens Please refer to Appendix C for a list of current known extensions at theclient's identity. "confidential": Clients that have credentials and their identity has been confirmed bytime of this publication. 1.8. Compatibility with OAuth 2.0 OAuth 2.1 is compatible with OAuth 2.0 with theAS are designated as "confidential clients" "credentialed": Clients that have credentialsextensions andtheir identity has beenrestrictions from known best current practices applied. Specifically, features notbeen confirmed by the AS are designatedspecified in OAuth 2.0 core, such as"credentialed clients" "public": Clients without credentialsPKCE, arecalled "public clients" Any clients with credentials MUST take precautions to prevent leakage and abuse of their credentials. Authorization servers SHOULD consider the level of confidencerequired ina client's identity when deciding whether they allow such a client access to more critical functions,OAuth 2.1. Additionally, some features available in OAuth 2.0, such as theClientImplicit or Resource Owner Credentials granttype. A single "client_id" MUST NOT be treated as more than one type of client. This specification has been designed around the following client profiles: "web application": A web application is a confidential client running on a web server. Resource owners access the client via an HTML user interface renderedtypes, are not specified ina user-agent onOAuth 2.1. Furthermore, some behaviors allowed in OAuth 2.0 are restricted in OAuth 2.1, such as thedevice usedstrict string matching of redirect URIs required by OAuth 2.1. See Section 10 for more details on theresource owner.differences from OAuth 2.0. 1.9. Notational Conventions Theclient credentials as well as any access token issued to the clientkey words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document arestored on the web serverto be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234]. Additionally, the rule URI-reference is included from "Uniform Resource Identifier (URI): Generic Syntax" [RFC3986]. Certain security-related terms arenot exposedtoor accessible bybe understood in theresource owner. "browser-based application": A browser-based applicationsense defined in [RFC4949]. These terms include, but are not limited to, "attack", "authentication", "authorization", "certificate", "confidentiality", "credential", "encryption", "identity", "sign", "signature", "trust", "validate", and "verify". The term "payload" isa public clientto be interpreted as described inwhich the client codeSection 3.3 of [RFC7231]. The term "user agent" isdownloaded from a web server and executes within a user-agent (e.g., web browser) on the device used byto be interpreted as described in [RFC7230]. Unless otherwise noted, all theresource owner. Protocol dataprotocol parameter names andcredentialsvalues areeasily accessible (and often visible) to the resource owner. Since such applications reside withincase sensitive. 2. Client Registration Before initiating theuser-agent, they can make seamless use ofprotocol, theuser-agent capabilities when requesting authorization. "native application": A native application is a publicclientinstalled and executed onmust establish its registration with thedevice used byauthorization server. The means through which theresource owner. Protocol data and credentialsclient registers with the authorization server areaccessible tobeyond the scope of this specification but typically involve theresource owner. It is assumed that anyclientauthentication credentials included indeveloper manually registering theapplication can be extracted. Onclient at theother hand, dynamically issued credentials such as access tokens or refresh tokens can receiveauthorization server's website after creating anacceptable levelaccount and agreeing to the service's Terms ofprotection. AtService, or by using Dynamic Client Registration ([RFC7591]). Client registration does not require aminimum, these credentials are protected from hostile servers with whichdirect interaction between theapplication may interact. On some platforms, these credentials might be protected from other applications residing onclient and thesame device. 2.2. Client Identifier Theauthorizationserver issuesserver. When supported by theregisteredauthorization server, registration can rely on other means for establishing trust and obtaining the required clientaproperties (e.g., redirect URI, clientidentifier - a unique string representing thetype). For example, registrationinformation providedcan be accomplished using a self-issued or third-party-issued assertion, or by theclient. Theauthorization server performing clientidentifier is notdiscovery using asecret; it is exposed totrusted channel. When registering a client, theresource owner and MUST NOT be used alone for client authentication. Theclientidentifier is unique todeveloper SHALL: * specify theauthorization server. Theclientidentifier string size is left undefined by this specification. Thetype as described in Section 2.1, * provide clientshould avoid making assumptions aboutdetails needed by the grant type in use, such as redirect URIs as described in Section 2.3, and * include any other information required by theidentifier size. Theauthorization serverSHOULD document(e.g., application name, website, description, logo image, thesizeacceptance ofany identifier it issues. Authorization servers SHOULD NOT allowlegal terms). Dynamic Client Registration ([RFC7591]) defines a common general data model for clientsto choose or influence their "client_id" value. See Section 9.6 for details. 2.3.that may be used even with manual client registration. 2.1. ClientAuthentication Confidential and credentialed clients establish aTypes OAuth 2.1 defines three clientauthentication methodtypes based on their ability to authenticate securely with the authorization serversuitable for the security requirements ofas well as the authorizationserver. The authorization server MAY accept any formserver's assurance ofclient authentication meeting its security requirements. Confidentialthe client's identity. "confidential": Clients that have credentials andcredentialed clientstheir identity has been confirmed by the AS aretypically issued (or establish) a set of clientdesignated as "confidential clients" "credentialed": Clients that have credentialsused for authenticating withand their identity has been not been confirmed by theauthorization server (e.g., password, public/private key pair).AS are designated as "credentialed clients" "public": Clients without credentials are called "public clients" Any clients with credentials MUST take precautions to prevent leakage and abuse of their credentials. Authorization servers SHOULDuseconsider the level of confidence in a client's identity when deciding whether they allow such a clientauthentication if possible. It is RECOMMENDEDaccess touse asymmetric (public-key based) methods for client authenticationmore critical functions, such asmTLS [RFC8705] or "private_key_jwt" [OpenID]. When asymmetric methods for client authentication are used, authorization servers do not need to store sensitive symmetric keys, making these methodsthe Client Credentials grant type. A single "client_id" MUST NOT be treated as morerobust against a numberthan one type ofattacks. The authorization server MAY establishclient. This specification has been designed around the following client profiles: "web application": A web application is a confidential clientauthentication method with public clients, which converts them to credentialed clients. However, the authorization server MUST NOT relyrunning oncredentialed client authentication for the purpose of identifyinga web server. Resource owners access theclient. TheclientMUST NOT use more than one authentication method in each request. 2.3.1. Client Secret Clientsvia an HTML user interface rendered inpossession ofa user agent on the device used by the resource owner. The clientsecret, sometimes knowncredentials asa client password, MAY use the HTTP Basic authentication schemewell asdefined in [RFC2617]any access tokens issued toauthenticate withtheauthorization server. Theclientidentifier is encoded usingare stored on the"application/x-www-form-urlencoded" encoding algorithm per Appendix B,web server and are not exposed to or accessible by theencoded valueresource owner. "browser-based application": A browser-based application isused as the username;a public client in which the clientsecretcode isencoded using the same algorithmdownloaded from a web server and executes within a user agent (e.g., web browser) on the device usedasby thepassword. The authorization server MUST supportresource owner. Protocol data and credentials are easily accessible (and often visible) to theHTTP Basic authentication scheme for authenticating clients that were issuedresource owner. Since such applications reside within the user agent, they can make seamless use of the user agent capabilities when requesting authorization. "native application": A native application is a public clientsecret. For example (with extra line breaks for display purposes only): Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3 Alternatively,installed and executed on theauthorization server MAY support includingdevice used by theclientresource owner. Protocol data and credentialsinare accessible to therequest-body using the following parameters: "client_id": REQUIRED. Theresource owner. It is assumed that any clientidentifierauthentication credentials included in the application can be extracted. On the other hand, dynamically issuedtocredentials such as access tokens or refresh tokens can receive an acceptable level of protection. At a minimum, these credentials are protected from hostile servers with which theclient duringapplication may interact. On some platforms, these credentials might be protected from other applications residing on theregistration process described by Sectionsame device. 2.2."client_secret": REQUIRED.Client Identifier Theclient secret. Includingauthorization server issues the registered clientcredentials ina client identifier - a unique string representing therequest-body usingregistration information provided by thetwo parametersclient. The client identifier isNOT RECOMMENDED and SHOULD be limited to clients unablenot a secret; it is exposed todirectly utilize the HTTP Basic authentication scheme (or other password-based HTTP authentication schemes). The parameters can only be transmitted intherequest-bodyresource owner and MUST NOT beincluded in the request URI. For example, a request to refresh an access token (Section 6) using the body parameters (with extra line breaksused alone fordisplay purposes only): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA &client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIwclient authentication. Theauthorization server MUST requireclient identifier is unique to theuse of TLS as described in Section 1.6 when sending requests using password authentication. Sinceauthorization server. The client identifier string size is left undefined by this specification. The clientauthentication method involves a password,should avoid making assumptions about the identifier size. The authorization serverMUST protectSHOULD document the size of anyendpoint utilizingidentifier itagainst brute force attacks. 2.3.2. Other Authentication Methodsissues. Authorization servers SHOULD NOT allow clients to choose or influence their "client_id" value. See Section 7.6 for details. 2.3. Client Redirection Endpoint Theauthorization server MAY support any suitable authentication scheme matching its security requirements. When using other authentication methods, the authorization server MUST define a mapping between theclientidentifier (registration record) and authentication scheme. Some additional authentication methods such as mTLS [RFC8705] and "private_key_jwt" [OpenID] are defined in the "OAuth Token Endpoint Authentication Methods (https://www.iana.org/assignments/oauth- parameters/oauth-parameters.xhtml#token-endpoint-auth-method)" registry, and may be usefulredirection endpoint (also referred to asgeneric client authentication methods beyond the specific use of protecting the token endpoint. 2.4. Unregistered Clients This specification does not exclude"redirect endpoint") is theuseURI ofunregistered clients. However,theuse of such clients is beyondclient that thescope of this specification and requires additional security analysis and review of its interoperability impact. 3. Protocol Endpoints The authorization process utilizes twoauthorization serverendpoints (HTTP resources): * Authorization endpoint - used byredirects theclientuser agent back toobtain authorization fromafter completing its interaction with the resourceowner via user-agent redirection. * Token endpoint - used byowner. The authorization server redirects theclientuser agent toexchange an authorization grant for an access token, typically with client authentication. As well asoneclient endpoint: * Redirection endpoint - used byof the client's redirection endpoints previously established with the authorization serverto return responses containing authorization credentials toduring the clientvia the resource owner user-agent. Not every authorization grant type utilizes both endpoints. Extension grant types MAY define additional endpoints as needed. 3.1. Authorization Endpointregistration process. Theauthorization endpoint is used to interact with the resource owner and obtain an authorization grant. The authorization serverredirect URI MUSTfirst verify the identity of the resource owner. The way in which the authorization server authenticates the resource owner (e.g., username and password login, session cookies) is beyond the scope of this specification. The means through which the client obtains the location of the authorization endpoint are beyond the scope of this specification, but the location is typically provided in the service documentation, or in the authorization server's metadata document ([RFC8414]).be an absolute URI as defined by [RFC3986] Section 4.3. The endpoint URI MAY include an"application/x-www-form-urlencoded""application/x-www- form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4), which MUST be retained when adding additional query parameters. The endpoint URI MUST NOT include a fragment component.Since requests to the authorization2.3.1. Endpoint Request Confidentiality The redirection endpointresult in user authentication and the transmission of clear-text credentials (in the HTTP response), the authorization server MUSTSHOULD require the use of TLS as described in Section1.61.5 whensending requests to the authorization endpoint. The authorization server MUST support the use oftheHTTP "GET" method [RFC7231] forrequested response type is "code", or when theauthorization endpoint and MAY supportredirection request will result in theusetransmission of sensitive credentials over an open network. If TLS is not available, the"POST" method as well. Theauthorization serverMUST ignore unrecognized request parameters. Request and response parameters defined by this specification MUST NOT be included more than once. Parameters sent without a value MUST be treated as if they were omitted fromSHOULD warn therequest. 3.1.1. Response Type The authorization endpoint is used byresource owner about theauthorization code flow. The client informsinsecure endpoint prior to redirection (e.g., display a message during the authorizationserver of the desired response type using the following parameter: "response_type": REQUIRED. The valuerequest). 2.3.2. Registration Requirements Authorization servers MUSTbe "code" for requesting anrequire clients to register their complete redirect URI (including the path component) and reject authorizationcode as described by Section 4.1.1, or a registered extension value as described by Section 8.4. Extension response types MAY containrequests that specify aspace-delimited (%x20) list of values, where the order of values does not matter (e.g., response type "a b" isredirect URI that doesn't exactly match one that was registered; thesame as "b a"). The meaning of such composite response typesexception isdefined by their respective specifications. Some extension response types are defined by ([OpenID]). Ifloopback redirects, where anauthorization request is missing the "response_type" parameter, or if the response typeexact match isnot understood,required except for the port URI component. For private-use URI scheme-based redirect URIs, authorizationserver MUST return an error response as describedservers SHOULD enforce the requirement in Section4.1.2.1. 3.1.2. Redirection Endpoint After completing its interaction with the resource owner, the authorization server directs8.3.1 that clients use schemes that are reverse domain name based. At a minimum, any private-use URI scheme that doesn't contain a period character (".") SHOULD be rejected. The client MAY use theresource owner's user-agent back"state" request parameter to achieve per- request customization if needed rather than varying theclient.redirect URI per request. The authorization serverredirectsMAY allow theuser-agentclient tooneregister multiple redirect URIs. Without requiring registration ofthe client's redirection endpoints previously established withredirect URIs, attackers can use the authorizationserver during the client registration process. The redirect URI MUST be an absolute URI as defined by [RFC3986] Section 4.3. The endpoint URI MAY include an "application/x-www- form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4), which MUST be retained when adding additional query parameters. The endpoint URI MUST NOT include a fragment component. 3.1.2.1. Endpoint Request Confidentiality The redirection endpoint SHOULD require the use of TLS as described in Section 1.6 when the requested response type is "code", or when the redirection request will result in the transmission of sensitive credentials over an open network. If TLS is not available, the authorization server SHOULD warn the resource owner about the insecure endpoint prior to redirection (e.g., display a message during the authorization request). Lack of transport-layer security can have a severe impact on the security of the client and the protected resources it is authorized to access. The use of transport-layer security is particularly critical when the authorization process is used as a form of delegated end-user authentication by the client (e.g., third-party sign-in service). 3.1.2.2. Registration Requirements The authorization server MUST require all clients to register one or more complete redirect URIs prior to utilizing the authorization endpoint. The client MAY use the "state" request parameter to achieve per-request customization if needed. The authorization server MAY allow the client to register multiple redirect URIs. Without requiring registration of redirect URIs, attackers can use the authorization endpointendpoint as an open redirector as described in Section9.18. 3.1.2.3. Dynamic Configuration7.18. 2.3.3. Multiple Redirect URIs If multiple redirect URIs have beenregisteredregistered, the client MUST include a redirect URI with the authorization request using the "redirect_uri" request parameter.3.1.2.4.2.3.4. Invalid Endpoint If an authorization request fails validation due to a missing, invalid, or mismatching redirect URI, the authorization server SHOULD inform the resource owner of the error and MUST NOT automatically redirect theuser-agentuser agent to the invalid redirect URI.3.1.2.5.2.3.5. Endpoint Content The redirection request to the client's endpoint typically results in an HTML document response, processed by theuser-agent.user agent. If the HTML response is served directly as the result of the redirection request, any script included in the HTML document will execute with full access to the redirect URI and the credentials (e.g. authorization code) it contains. Additionally, the request URL containing the authorization code may be sent in the HTTP Referer header to any embedded images, stylesheets and other elements loaded in the page. The client SHOULD NOT include any third-party scripts (e.g., third- party analytics, social plug-ins, ad networks) in the redirection endpoint response. Instead, it SHOULD extract the credentials from the URI and redirect theuser-agentuser agent again to another endpoint without exposing the credentials (in the URI or elsewhere). If third-party scripts are included, the client MUST ensure that its own scripts (used to extract and remove the credentials from the URI) will execute first.3.2. Token Endpoint The token endpoint is used by the client to obtain an access token using2.4. Client Authentication Confidential and credentialed clients establish agrant such as those described in Section 4 and Section 6. The means through which theclientobtains the location of the token endpoint are beyond the scope of this specification, butauthentication method with thelocation is typically provided inauthorization server suitable for theservice documentation and configured during developmentsecurity requirements of theclient, or provided in theauthorizationserver's metadata document ([RFC8414]) and fetched programmatically at runtime.server. Theendpoint URIauthorization server MAYinclude an "application/x-www-form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4)accept any form of client authentication meeting its security requirements. Confidential andMUST NOT includecredentialed clients are typically issued (or establish) afragment component. Since requests to the token endpoint result in the transmissionset ofclear-textclient credentials(in the HTTP request and response),used for authenticating with the authorization serverMUST require the use of TLS as described in Section 1.6 when sending requests to the token endpoint. The client MUST use the HTTP "POST" method when making access token requests.(e.g., password, public/private key pair). The authorization server MUSTignore unrecognized request parameters. Parameters sent without a value MUST be treated as if they were omitted fromauthenticate therequest. Request and response parameters defined by this specification MUST NOT be included more than once. 3.2.1. Client Authentication Confidential clients or other clients issuedclientcredentials MUST authenticate withwhenever possible. If the authorization serveras described in Section 2.3 when making requestscannot authenticate the client due to thetoken endpoint. Client authentication is used for: * Enforcingclient's nature, thebinding of refresh tokens andauthorizationcodesserver SHOULD utilize other means to protect resource owners from such potentially malicious clients. For example, theclient they were issued to. Client authentication is critical when anauthorizationcode is transmittedserver can engage the resource owner to assist in identifying theredirection endpoint over an insecure channel. * Recovering from a compromisedclientby disabling theand its origin. It is RECOMMENDED to use asymmetric (public-key based) methods for client authentication such as mTLS [RFC8705] orchanging its credentials, thus preventing an attacker from abusing stolen refresh tokens. Changing a single set of"private_key_jwt" [OpenID]. When asymmetric methods for clientcredentials is significantly faster than revoking an entire set of refresh tokens. * Implementingauthenticationmanagement best practices, which require periodic credential rotation. Rotation of an entire set of refresh tokens can be challenging, while rotation ofare used, authorization servers do not need to store sensitive symmetric keys, making these methods more robust against asingle setnumber ofclient credentials is significantly easier. 3.3. Access Token Scopeattacks. The authorizationand token endpoints allow theserver MAY establish a client authentication method with public clients, which converts them tospecify the scope of the access request using the "scope" request parameter. In turn,credentialed clients. However, the authorization serveruses the "scope" response parameter to inform theMUST NOT rely on credentialed clientofauthentication for thescopepurpose of identifying theaccess token issued.client. Thevalueclient MUST NOT use more than one authentication method in each request. 2.4.1. Client Secret Clients in possession ofthe scope parameter is expresseda client secret, sometimes known as alist of space- delimited, case-sensitive strings. The strings areclient password, MAY use the HTTP Basic authentication scheme as definedbyin [RFC7235] to authenticate with the authorization server.If the value contains multiple space-delimited strings, their order does not matter, and each string adds an additional access range to the requested scope. scope = scope-token *( SP scope-token ) scope-token = 1*( %x21 / %x23-5B / %x5D-7E )Theauthorization server MAY fully or partially ignore the scope requested byclient identifier is encoded using theclient, based on"application/x-www-form-urlencoded" encoding algorithm per Appendix B, and theauthorization server policy orencoded value is used as theresource owner's instructions. Ifusername; theissued access token scopeclient secret isdifferent from the one requested byencoded using theclient,same algorithm and used as the password. The authorization server MUSTincludesupport the"scope" response parameterHTTP Basic authentication scheme for authenticating clients that were issued a client secret. For example (with extra line breaks for display purposes only): Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3 In addition toinformthat, the authorization server MAY support including the clientofcredentials in theactual scope granted. Ifrequest-body using the following parameters: "client_id": REQUIRED. The clientomitsidentifier issued to thescope parameter when requesting authorization,client during theauthorization server MUST eitherregistration processthe request using a pre-defined default value or fail the request indicating an invalid scope.described by Section 2.2. "client_secret": REQUIRED. Theauthorization server SHOULD document its scope requirements and default value (if defined). 4. Obtaining Authorization To request an access token,client secret. Including the clientobtains authorization fromcredentials in the request-body using theresource owner. OAuth definestwoauthorization grant types: authorization code and client credentials. It also provides an extension mechanism for defining additional grant types. 4.1. Authorization Code Grant The authorization code grant typeparameters isused toNOT RECOMMENDED and SHOULD be limited to clients unable to directly utilize the HTTP Basic authentication scheme (or other password-based HTTP authentication schemes). The parameters can only be transmitted in the request-body and MUST NOT be included in the request URI. For example, a request to refresh an access token (Section 4.3) using the body parameters (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA &client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw The authorization server MUST require the use of TLS as described in Section 1.5 when sending requests using password authentication. Since this client authentication method involves a password, the authorization server MUST protect any endpoint utilizing it against brute force attacks. 2.4.2. Other Authentication Methods The authorization server MAY support any suitable authentication scheme matching its security requirements. When using other authentication methods, the authorization server MUST define a mapping between the client identifier (registration record) and authentication scheme. Some additional authentication methods such as mTLS [RFC8705] and "private_key_jwt" [OpenID] are defined in the "OAuth Token Endpoint Authentication Methods (https://www.iana.org/assignments/oauth- parameters/oauth-parameters.xhtml#token-endpoint-auth-method)" registry, and may be useful as generic client authentication methods beyond the specific use of protecting the token endpoint. 2.5. Unregistered Clients This specification does not exclude the use of unregistered clients. However, the use of such clients is beyond the scope of this specification and requires additional security analysis and review of its interoperability impact. 3. Protocol Endpoints The authorization process utilizes two authorization server endpoints (HTTP resources): * Authorization endpoint - used by the client to obtain authorization from the resource owner via user agent redirection. * Token endpoint - used by the client to exchange an authorization grant for an access token, typically with client authentication. As well as one client endpoint: * Redirection endpoint - used by the authorization server to return responses containing authorization credentials to the client via the resource owner user agent. Not every authorization grant type utilizes both endpoints. Extension grant types MAY define additional endpoints as needed. 3.1. Authorization Endpoint The authorization endpoint is used to interact with the resource owner and obtain an authorization grant. The authorization server MUST first verify the identity of the resource owner. The way in which the authorization server authenticates the resource owner (e.g., username and password login, session cookies) is beyond the scope of this specification. The means through which the client obtains the location of the authorization endpoint are beyond the scope of this specification, but the location is typically provided in the service documentation, or in the authorization server's metadata document ([RFC8414]). The endpoint URI MAY include an "application/x-www-form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4), which MUST be retained when adding additional query parameters. The endpoint URI MUST NOT include a fragment component. Since requests to the authorization endpoint result in user authentication and the transmission of clear-text credentials (in the HTTP response), the authorization server MUST require the use of TLS as described in Section 1.5 when sending requests to the authorization endpoint. The authorization server MUST support the use of the HTTP "GET" method [RFC7231] for the authorization endpoint and MAY support the use of the "POST" method as well. The authorization server MUST ignore unrecognized request parameters. Request and response parameters defined by this specification MUST NOT be included more than once. Parameters sent without a value MUST be treated as if they were omitted from the request. 3.2. Token Endpoint The token endpoint is used by the client to obtainbothan access token using a grant such as those described in Section 4 and Section 4.3. The means through which the client obtains the location of the token endpoint are beyond the scope of this specification, but the location is typically provided in the service documentation and configured during development of the client, or provided in the authorization server's metadata document ([RFC8414]) and fetched programmatically at runtime. The endpoint URI MAY include an "application/x-www-form-urlencoded" formatted (per Appendix B) query component ([RFC3986] Section 3.4) and MUST NOT include a fragment component. Since requests to the token endpoint result in the transmission of clear-text credentials (in the HTTP request and response), the authorization server MUST require the use of TLS as described in Section 1.5 when sending requests to the token endpoint. The client MUST use the HTTP "POST" method when making access token requests. The authorization server MUST ignore unrecognized request parameters. Parameters sent without a value MUST be treated as if they were omitted from the request. Request and response parameters defined by this specification MUST NOT be included more than once. 3.2.1. Client Authentication Confidential or credentialed clients MUST authenticate with the authorization server as described in Section 2.4 when making requests to the token endpoint. Client authentication is used for: * Enforcing the binding of refresh tokens and authorization codes to the client they were issued to. Client authentication adds an additional layer of security when an authorization code is transmitted to the redirection endpoint over an insecure channel. * Recovering from a compromised client by disabling the client or changing its credentials, thus preventing an attacker from abusing stolen refresh tokens.Since thisChanging a single set of client credentials is significantly faster than revoking an entire set of refresh tokens. * Implementing authentication management best practices, which require periodic credential rotation. Rotation of an entire set of refresh tokens can be challenging, while rotation of a single set of client credentials is significantly easier. 3.2.2. Token Request The client makes aredirect-based flow,request to the token endpoint by sending the following parameters using the "application/x-www-form-urlencoded" format per Appendix B with a character encoding of UTF-8 in the HTTP request payload: "client_id": REQUIRED, if the client is not authenticating with the authorization server as described in Section 3.2.1. "scope": OPTIONAL. The scope of theclient must be capableaccess request as described by Section 3.2.2.1. "grant_type": REQUIRED. Identifier ofinitiatingtheflowgrant type the client uses with theresource owner's user-agent (typically a web browser)particular token request. This specification defines the values "authorization_code", "refresh_token", andcapable of being redirected back to from"client_credentials". The grant type determines theauthorization server. +----------+ | Resource | | Owner | | | +----------+ ^ | (2) +----|-----+ Client Identifier +---------------+ | -+----(1)-- & Redirect URI ---->| | | User- | | Authorization | | Agent -+----(2)-- User authenticates --->| Server | | | | | | -+----(3)-- Authorization Code ---<| | +-|----|---+ +---------------+ | | ^ v (1) (3) | | | | | | ^ v | | +---------+ | | | |>---(4)-- Authorization Code ---------' | | Client | & Redirect URI | | | | | |<---(5)----- Access Token -------------------' +---------+ (w/ Optional Refresh Token) Note:further parameters required or supported by the token request. Thelines illustrating steps (1), (2), and (3)details of those grant types arebroken into two parts as they pass throughdefined below. Confidential or credentialed clients MUST authenticate with theuser-agent. Figure 3: Authorization Code Flow The flow illustratedauthorization server as described inFigure 3 includesSection 3.2.1. For example, the client makes the followingsteps: (1)HTTP request using TLS (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bed The authorization server MUST: * require clientinitiates the flow by directing the resource owner's user-agent toauthentication for confidential and credentialed clients (or clients with other authentication requirements), * authenticate theauthorization endpoint. Theclientincludes itsif clientidentifier, code challenge (derived from a generated code verifier), optional requested scope, optional local state,authentication is included Further grant type specific processing rules apply anda redirect URI to which the authorization server will sendare specified with theuser- agent back once access is granted (or denied). (2)respective grant type. 3.2.2.1. Access Token Scope The authorizationserver authenticates the resource owner (via the user-agent)andestablishes whethertoken endpoints allow theresource owner grants or deniesclient to specify the scope of theclient'saccessrequest. (3) Assumingrequest using theresource owner grants access,"scope" request parameter. In turn, the authorization serverredirectsuses theuser-agent back"scope" response parameter to inform the clientusingof theredirect URI provided earlier (inscope of therequest or during client registration).access token issued. Theredirect URI includes anvalue of the scope parameter is expressed as a list of space- delimited, case-sensitive strings. The strings are defined by the authorizationcodeserver. If the value contains multiple space-delimited strings, their order does not matter, andany local state provided byeach string adds an additional access range to theclient earlier. (4)requested scope. scope = scope-token *( SP scope-token ) scope-token = 1*( %x21 / %x23-5B / %x5D-7E ) Theclient requests anauthorization server MAY fully or partially ignore the scope requested by the client, based on the authorization server policy or the resource owner's instructions. If the issued access token scope is different from theauthorization server's token endpointone requested byincludingtheauthorization code received inclient, theprevious step, and including its code verifier. When makingauthorization server MUST include therequest,"scope" response parameter to inform the clientauthenticates withof the actual scope granted. If theauthorization server if it can. Theclientincludesomits theredirect URI used to obtainscope parameter when requesting authorization, the authorizationcode for verification. (5) The authorizationserverauthenticatesMUST either process theclient when possible, validatesrequest using a pre-defined default value or fail the request indicating an invalid scope. The authorizationcode, validates the code verifier,server SHOULD document its scope requirements andensures that the redirect URI received matches the URI used to redirect the client in step (3).default value (if defined). 3.2.3. Token Response Ifvalid,the access token request is valid and authorized, the authorization serverresponds back withissues an access tokenand, optionally, aand optional refresh token.4.1.1. Authorization Request To begin the authorization request,If the request clientbuildsauthentication failed or is invalid, the authorizationrequest URIserver returns an error response as described in Section 3.2.3.1. The authorization server issues an access token and optional refresh token byaddingcreating an HTTP response body using the "application/json" media type as defined by [RFC8259] with the following parameterstoand an HTTP 200 (OK) status code: "access_token": REQUIRED. The access token issued by the authorizationserver's authorization endpoint URI.server. "token_type": REQUIRED. Theclient will eventually redirect the user-agent to this URI to initiatetype of therequest,access token issued as described in Section4.1.1.1. Clients use a unique secret per authorization request to protect against authorization code injection and CSRF attacks.5.1. Value is case insensitive. "expires_in": RECOMMENDED. Theclient first generates this secret, which it can use at the timelifetime in seconds ofredeeming the authorization code to prove that the client usingtheauthorization code isaccess token. For example, thesame clientvalue "3600" denotes thatrequested it. 4.1.1.1. Client Initiates the Authorization Request The client constructstherequest URI by addingaccess token will expire in one hour from thefollowing parameters totime thequery component ofresponse was generated. If omitted, the authorizationendpoint URI usingserver SHOULD provide the"application/x-www-form-urlencoded" format, per Appendix B: "response_type": REQUIRED. Value MUST be set to "code". "client_id": REQUIRED. The client identifier as described in Section 2.2. "code_challenge": REQUIREDexpiration time via other means orRECOMMENDED (see Section 9.8). Code challenge. "code_challenge_method":document the default value. "scope": OPTIONAL,defaults to "plain"ifnot present inidentical to therequest. Code verifier transformation method is "S256" or "plain". "redirect_uri": OPTIONAL. As described in Section 3.1.2. "scope": OPTIONAL.scope requested by the client; otherwise, REQUIRED. The scope of the accessrequesttoken as described by Section3.3. "state":3.2.2.1. "refresh_token": OPTIONAL.An opaque valueThe refresh token, which can be usedby the clienttomaintain state betweenobtain new access tokens based on therequestgrant passed in the corresponding token request. Authorization servers SHOULD determine, based on a risk assessment andcallback. Thetheir own policies, whether to issue refresh tokens to a certain client. If the authorization serverincludes this value when redirectingdecides not to issue refresh tokens, the client MAY obtain new access tokens by starting the OAuth flow over, for example initiating a new authorization code request. In such a case, the authorization server may utilize cookies and persistent grants to optimize the user experience. If refresh tokens are issued, those refresh tokens MUST be bound to the scope and resource servers as consented by the resource owner. This is to prevent privilege escalation by theuser-agent back tolegitimate client and reduce theclient.impact of refresh token leakage. The"code_verifier" isparameters are serialized into aunique high-entropy cryptographically random string generated forJavaScript Object Notation (JSON) structure by adding eachauthorization request, usingparameter at theunreserved characters "[A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~"", with a minimum lengthhighest structure level. Parameter names and string values are included as JSON strings. Numerical values are included as JSON numbers. The order of43 charactersparameters does not matter and can vary. The authorization server MUST include the HTTP "Cache-Control" response header field [RFC7234] with amaximum lengthvalue of128 characters."no-store" in any response containing tokens, credentials, or other sensitive information. For example: HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-store { "access_token":"2YotnFZFEjr1zCsicMWpAA", "token_type":"Bearer", "expires_in":3600, "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA", "example_parameter":"example_value" } The clientstores the "code_verifier" temporarily, and calculates the "code_challenge" which it usesMUST ignore unrecognized value names in theauthorization request. ABNF for "code_verifier" is as follows. code-verifier = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39 NOTE:response. Thecode verifier SHOULD have enough entropy to make it impractical to guess the value. It is RECOMMENDED that the outputsizes ofa suitable random number generator be used to create a 32-octet sequence. The octet sequence is then base64url-encoded to produce a 43-octet URL-safe string to use astokens and other values received from thecode verifier.authorization server are left undefined. The clientthen creates a "code_challenge" derived fromshould avoid making assumptions about value sizes. The authorization server SHOULD document thecode verifier by using onesize of any value it issues. 3.2.3.1. Error Response The authorization server responds with an HTTP 400 (Bad Request) status code (unless specified otherwise) and includes the followingtransformations onparameters with the response: "error": REQUIRED. A single ASCII [USASCII] error codeverifier: S256 code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier))) plain code_challenge = code_verifier Iffrom theclient is capable of using "S256", it MUST use "S256", as "S256"following: "invalid_request": The request isMandatory To Implement (MTI) on the server. Clients are permitted to use "plain" only if they cannot support "S256" for some technical reason, for example constrained environments that do not havemissing ahashing function available, and know via out-of-band configuration or via Authorization Server Metadata ([RFC8414]) that the server supports "plain". ABNFrequired parameter, includes an unsupported parameter value (other than grant type), repeats a parameter, includes multiple credentials, utilizes more than one mechanism for authenticating the client, contains a "code_verifier" although no "code_challenge" was sent in the authorization request, or isas follows. code-challenge = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39otherwise malformed. "invalid_client": Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method). Theproperties "code_challenge" and "code_verifier"authorization server MAY return an HTTP 401 (Unauthorized) status code to indicate which HTTP authentication schemes areadopted fromsupported. If the client attempted to authenticate via the "Authorization" request header field, theOAuth 2.0 extension known as "Proof-Key for Code Exchange", or PKCE ([RFC7636]) where this technique was originally developed. Clients MUST use "code_challenge" and "code_verifier" andauthorizationserversserver MUSTenforce their use except under the conditions described in Section 9.8. In this case, using and enforcing "code_challenge"respond with an HTTP 401 (Unauthorized) status code and"code_verifier" as described ininclude thefollowing is still RECOMMENDED. The client directs"WWW-Authenticate" response header field matching the authentication scheme used by the client. "invalid_grant": The provided authorization grant (e.g., authorization code, resource ownertocredentials) or refresh token is invalid, expired, revoked, does not match theconstructedredirect URIusing an HTTP redirection,used in the authorization request, orby other means availablewas issued toit via the user- agent. For example, theanother client. "unauthorized_client": The authenticated clientdirects the user-agentis not authorized tomake the following HTTP request using TLS (with extra line breaks for display purposes only): GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_challenge=6fdkQaPm51l13DSukcAH3Mdx7_ntecHYd1vi3n0hMZY &code_challenge_method=S256 HTTP/1.1 Host: server.example.comuse this authorization grant type. "unsupported_grant_type": The authorizationserver validates the request to ensure that all required parameters are present and valid. In particular,grant type is not supported by the authorizationserver MUST validateserver. "invalid_scope": The requested scope is invalid, unknown, malformed, or exceeds the"redirect_uri" inscope granted by therequest if present, ensuring that it matches one ofresource owner. Values for theregistered redirect URIs previously established during client registration (Section 2). When comparing"error" parameter MUST NOT include characters outside thetwo URIsset %x20-21 / %x23-5B / %x5D-7E. "error_description": OPTIONAL. Human-readable ASCII [USASCII] text providing additional information, used to assist theauthorization server MUST using simple character-by-character string comparison as definedclient developer in[RFC3986], Section 6.2.1. If the request is valid,understanding theauthorization server authenticateserror that occurred. Values for theresource owner and obtains an authorization decision (by asking"error_description" parameter MUST NOT include characters outside theresource owner or by establishing approval via other means). Whenset %x20-21 / %x23-5B / %x5D-7E. "error_uri": OPTIONAL. A URI identifying adecision is established, the authorization server directshuman-readable web page with information about theuser-agenterror, used to provide theprovidedclientredirect URI using an HTTP redirection response, or by other means available to it via the user- agent. 4.1.2. Authorization Response If the resource owner grantsdeveloper with additional information about theaccess request,error. Values for theauthorization server issues an authorization code and delivers it"error_uri" parameter MUST conform to theclient by addingURI-reference syntax and thus MUST NOT include characters outside thefollowingset %x21 / %x23-5B / %x5D-7E. The parameterstoare included in thequery componentpayload of theredirect URIHTTP response using the"application/x-www-form-urlencoded" format, per Appendix B: "code": REQUIRED."application/json" media type as defined by [RFC7159]. Theauthorization code generatedparameters are serialized into a JSON structure by adding each parameter at theauthorization server.highest structure level. Parameter names and string values are included as JSON strings. Numerical values are included as JSON numbers. Theauthorization code MUST expire shortly after it is issued to mitigate the risk of leaks. A maximum authorization code lifetimeorder of10 minutes is RECOMMENDED. Theparameters does not matter and can vary. For example: HTTP/1.1 400 Bad Request Content-Type: application/json Cache-Control: no-store { "error":"invalid_request" } 4. Grant Types To request an access token, the clientMUST NOT useobtains authorization from the resource owner. This specification defines the following authorization grant types: * authorization codemore than once. If* client credentials, and * refresh token It also provides an extension mechanism for defining additional grant types. 4.1. Authorization Code Grant The authorization code grant type is usedmore than once, the authorization server MUST deny the request and SHOULD revoke (when possible) allto obtain both access tokens and refreshtokens previously issued based on that authorization code.tokens. The grant type uses the additional authorizationcode is boundendpoint to let theclient identifier and redirect URI. "state": REQUIRED ifauthorization server interact with the"state" parameter was presentresource owner in order to get consent for resource access. Since this is a redirect-based flow, the clientauthorization request. The exact value received frommust be capable of initiating theclient. For example,flow with the resource owner's user agent (typically a web browser) and capable of being redirected back to from the authorizationserver redirectsserver. +----------+ | Resource | | Owner | | | +----------+ ^ | (2) +----|-----+ Client Identifier +---------------+ | -+----(1)-- & Redirect URI ---->| | | User- | | Authorization | | Agent -+----(2)-- User authenticates --->| Server | | | | | | -+----(3)-- Authorization Code ---<| | +-|----|---+ +---------------+ | | ^ v (1) (3) | | | | | | ^ v | | +---------+ | | | |>---(4)-- Authorization Code ---------' | | Client | & Redirect URI | | | | | |<---(5)----- Access Token -------------------' +---------+ (w/ Optional Refresh Token) Note: The lines illustrating steps (1), (2), and (3) are broken into two parts as they pass through theuser-agent by sendinguser agent. Figure 3: Authorization Code Flow The flow illustrated in Figure 3 includes the followingHTTP response: HTTP/1.1 302 Found Location: https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA &state=xyzsteps: (1) The clientMUST ignore unrecognized response parameters. The authorization code string size is left undefinedinitiates the flow bythis specification.directing the resource owner's user agent to the authorization endpoint. The clientshould avoid making assumptions aboutincludes its client identifier, codevalue sizes. Thechallenge (derived from a generated code verifier), optional requested scope, optional local state, and a redirect URI to which the authorization serverSHOULD documentwill send thesize of any value it issues.user agent back once access is granted (or denied). (2) The authorization serverMUST associateauthenticates the"code_challenge"resource owner (via the user agent) and"code_challenge_method" values withestablishes whether theissued authorization code soresource owner grants or denies thecode challenge can be verified later. The exact method thatclient's access request. (3) Assuming the resource owner grants access, the authorization serverusesredirects the user agent back toassociatethe"code_challenge" withclient using theissued code is out of scope for this specification.redirect URI provided earlier (in the request or during client registration). The redirect URI includes an authorization codechallenge could be stored on the serverandassociated withany local state provided by thecode there.client earlier. (4) The"code_challenge" and "code_challenge_method" values may be stored in encrypted form inclient requests an access token from the authorization server's token endpoint by including the authorization codeitself, butreceived in theserver MUST NOT includeprevious step, and including its code verifier. When making the"code_challenge" value in a response parameter in a form that entities other thanrequest, theAS can extract. 4.1.2.1. Error Response Ifclient authenticates with therequest fails due to a missing, invalid, or mismatching redirect URI, orauthorization server iftheit can. The clientidentifier is missing or invalid,includes the redirect URI used to obtain the authorization code for verification. (5) The authorization serverSHOULD informauthenticates theresource owner ofclient when possible, validates theerrorauthorization code, validates the code verifier, andMUST NOT automaticallyensures that the redirect URI received matches theuser-agentURI used tothe invalidredirectURI. An AS MUST reject requests without a "code_challenge" from public clients, and MUST reject such requests from other clients unless there is reasonable assurance thatthe clientmitigates authorization code injectioninother ways. See Section 9.8 for details.step (3). If valid, the authorization serverdoes not supportresponds back with an access token and, optionally, a refresh token. 4.1.1. Authorization Request To begin therequested "code_challenge_method" transformation,authorization request, the client builds the authorization request URI by adding parameters to the authorization server's authorization endpointMUST returnURI. The client will eventually redirect the user agent to this URI to initiate the request. Clients use a unique secret per authorizationerror response with "error" value setrequest to"invalid_request".protect against authorization code injection and CSRF attacks. The"error_description" or the response of "error_uri" SHOULD explainclient first generates this secret, which it can use at thenaturetime oferror, e.g., transform algorithm not supported. If the resource owner deniesredeeming theaccess request or ifauthorization code to prove that therequest fails for reasons other than a missing or invalid redirect URI,client using the authorizationserver informscode is the same client that requested it. The client constructs the request URI by adding the following parameters to the query component of theredirectauthorization endpoint URI using the "application/x-www-form-urlencoded" format, per Appendix B:"error":"response_type": REQUIRED.A single ASCII [USASCII] error code from the following: "invalid_request":The authorization endpoint supports different sets of requestis missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. "unauthorized_client":and response pameters. The clientis not authorizeddetermines the type of flow by using a certain "response_type" value. This specification defines the value "code", which must be used torequest ansignal that the client wants to use the authorization codeusing this method. "access_denied": The resource owner or authorization server deniedflow. Extension response types MAY contain a space-delimited (%x20) list of values, where therequest. "unsupported_response_type": The authorization serverorder of values does notsupport obtainingmatter (e.g., response type "a b" is the same as "b a"). The meaning of such composite response types is defined by their respective specifications. Some extension response types are defined by ([OpenID]). If an authorizationcode using this method. "invalid_scope": The requested scoperequest isinvalid, unknown,missing the "response_type" parameter, ormalformed. "server_error": Theif the response type is not understood, the authorization serverencounteredMUST return anunexpected condition that prevented it from fulfillingerror response as described in Section 4.1.2.1. "client_id": REQUIRED. The client identifier as described in Section 2.2. "code_challenge": REQUIRED or RECOMMENDED (see Section 7.8). Code challenge. "code_challenge_method": OPTIONAL, defaults to "plain" if not present in the request.(This error codeCode verifier transformation method isneeded because a 500 Internal Server Error HTTP status code cannot be returned to"S256" or "plain". "redirect_uri": OPTIONAL. As described in Section 2.3. "scope": OPTIONAL. The scope of the access request as described by Section 3.2.2.1. "state": OPTIONAL. An opaque value used by the clientvia an HTTP redirect.) "temporarily_unavailable":to maintain state between the request and callback. The authorization serveris currently unable to handleincludes this value when redirecting therequest dueuser agent back toa temporary overloading or maintenance oftheserver. (This error codeclient. The "code_verifier" isneeded becausea503 Service Unavailable HTTP status code cannot be returned to the client via an HTTP redirect.) Valuesunique high-entropy cryptographically random string generated for each authorization request, using the"error" parameter MUST NOT includeunreserved charactersoutside the set %x20-21"[A-Z] /%x23-5B[a-z] /%x5D-7E. "error_description": OPTIONAL. Human-readable ASCII [USASCII] text providing additional information, used to assist the client developer in understanding the error that occurred. Values for the "error_description" parameter MUST NOT include characters outside the set %x20-21[0-9] /%x23-5B"-" /%x5D-7E. "error_uri": OPTIONAL. A URI identifying a human-readable web page"." / "_" / "~"", withinformation about the error, used to provide thea minimum length of 43 characters and a maximum length of 128 characters. The clientdeveloper with additional information about the error. Values for the "error_uri" parameter MUST conform tostores theURI-reference syntax"code_verifier" temporarily, andthus MUST NOT include characters outsidecalculates theset %x21 / %x23-5B / %x5D-7E. "state": REQUIRED if a "state" parameter was present"code_challenge" which it uses in theclientauthorization request.The exact value received from the client. For example,ABNF for "code_verifier" is as follows. code-verifier = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39 NOTE: The code verifier SHOULD have enough entropy to make it impractical to guess theauthorization server redirectsvalue. It is RECOMMENDED that theuser-agent by sendingoutput of a suitable random number generator be used to create a 32-octet sequence. The octet sequence is then base64url-encoded to produce a 43-octet URL-safe string to use as thefollowing HTTP response: HTTP/1.1 302 Found Location: https://client.example.com/cb?error=access_denied&state=xyz 4.1.3. Access Token Requestcode verifier. The clientmakesthen creates arequest to"code_challenge" derived from thetoken endpointcode verifier bysendingusing one of the followingparameters usingtransformations on the"application/x-www-form-urlencoded" format per Appendix B with a character encoding of UTF-8 incode verifier: S256 code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier))) plain code_challenge = code_verifier If theHTTP request payload: "grant_type": REQUIRED. Valueclient is capable of using "S256", it MUSTbe set to "authorization_code". "code": REQUIRED. The authorization code received fromuse "S256", as "S256" is Mandatory To Implement (MTI) on theauthorizationserver."redirect_uri": REQUIRED,Clients are permitted to use "plain" only if they cannot support "S256" for some technical reason, for example constrained environments that do not have a hashing function available, and know via out-of-band configuration or via Authorization Server Metadata ([RFC8414]) that the"redirect_uri" parameter was included inserver supports "plain". ABNF for "code_challenge" is as follows. code-challenge = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39 The properties "code_challenge" and "code_verifier" are adopted from theauthorization requestOAuth 2.0 extension known asdescribed in Section 4.1.1,"Proof-Key for Code Exchange", or PKCE ([RFC7636]) where this technique was originally developed. Clients MUST use "code_challenge" andtheir values"code_verifier" and authorization servers MUSTbe identical. "client_id": REQUIRED, if the client is not authenticating withenforce their use except under theauthorization server asconditions described in Section3.2.1. "code_verifier": REQUIRED, if the7.8. In this case, using and enforcing "code_challenge"parameter was includedand "code_verifier" as described in theauthorization request. MUST NOT be used otherwise.following is still RECOMMENDED. Theoriginal code verifier string. Confidentialclient directs the resource owner to the constructed URI using an HTTP redirection, orcredentialed clients MUST authenticate withby other means available to it via theauthorization server as described in Section 3.2.1.user agent. For example, the clientmakesdirects the user agent to make the following HTTP request using TLS (with extra line breaks for display purposes only):POST /tokenGET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_challenge=6fdkQaPm51l13DSukcAH3Mdx7_ntecHYd1vi3n0hMZY &code_challenge_method=S256 HTTP/1.1 Host: server.example.comAuthorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bedThe authorization serverMUST: * require client authentication for confidential and credentialed clients (or clients with other authentication requirements), * authenticate the client if client authentication is included, * ensure thatvalidates theauthorization code was issuedrequest tothe authenticated confidential or credentialed client, or if the client is public,ensure thatthe code was issued to "client_id" in the request, * verify that the authorization code is valid, * verify that the "code_verifier" parameter isall required parameters are presentifandonly if a "code_challenge" parameter was present invalid. In particular, the authorizationrequest, * if a "code_verifier" is present, verify the "code_verifier" by calculating the code challenge from the received "code_verifier" and comparing it with the previously associated "code_challenge", after first transforming it according to the "code_challenge_method" method specified by the client, and * ensure that the "redirect_uri" parameter is present ifserver MUST validate the "redirect_uri"parameter was includedin theinitial authorizationrequestas described in Section 4.1.1.1, andifincluded ensurepresent, ensuring thattheir values are identical. 4.1.4. Access Token Response Ifit matches one of theaccess token request is valid and authorized,registered redirect URIs previously established during client registration (Section 2). When comparing the two URIs the authorization serverissues an access token and optional refresh tokenMUST using simple character-by-character string comparison asdescribeddefined in [RFC3986], Section5.1.6.2.1. If the requestclient authentication failed orisinvalid,valid, the authorization serverreturns an error response as described in Section 5.2. An example successful response: HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-store Pragma: no-cache { "access_token": "2YotnFZFEjr1zCsicMWpAA", "token_type": "Bearer", "expires_in": 3600, "refresh_token": "tGzv3JOkF0XG5Qx2TlKWIA", "example_parameter": "example_value" } 4.2. Client Credentials Grant The client can request an access token using only its client credentials (or other supported means of authentication) whenauthenticates theclient is requesting access toresource owner and obtains an authorization decision (by asking theprotected resources under its control, or those of anotherresource ownerthat have been previously arranged withor by establishing approval via other means). When a decision is established, the authorization server(the method of which is beyonddirects thescope of this specification). Theuser agent to the provided clientcredentials grant type MUST only be used by confidentialredirect URI using an HTTP redirection response, orcredentialed clients. +---------+ +---------------+ | | | | | |>--(1)- Client Authentication --->|by other means available to it via the user agent. 4.1.2. Authorization| | Client | | Server | | |<--(2)---- Access Token ---------<| | | | | | +---------+ +---------------+ Figure 4: Client Credentials Flow The flow illustrated in Figure 4 includesResponse If thefollowing steps: (1) The client authenticates withresource owner grants theauthorization server and requests anaccesstoken fromrequest, thetoken endpoint. (2) Theauthorization serverauthenticates the client, and if valid,issues anaccess token. 4.2.1. Authorization Request and Response Since the client authentication is used as the authorization grant, no additionalauthorizationrequest is needed. 4.2.2. Access Token Request The client makes a requestcode and delivers it to thetoken endpointclient by adding the following parameters to the query component of the redirect URI using the "application/x-www-form-urlencoded"formatformat, per AppendixB with a character encoding of UTF-8 in the HTTP request payload: "grant_type":B: "code": REQUIRED.ValueThe authorization code generated by the authorization server. The authorization code MUSTbe setexpire shortly after it is issued to"client_credentials". "scope": OPTIONAL. The scopemitigate the risk of leaks. A maximum authorization code lifetime of 10 minutes is RECOMMENDED. The client MUST NOT use the authorization code more than once. If an authorization code is used more than once, the authorization server MUST deny theaccessrequestas describedand SHOULD revoke (when possible) all access tokens and refresh tokens previously issued based on that authorization code. The authorization code is bound to the client identifier and redirect URI. "state": REQUIRED if the "state" parameter was present in the client authorization request. The exact value received from the client. For example, the authorization server redirects the user agent bySection 3.3.sending the following HTTP response: HTTP/1.1 302 Found Location: https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA &state=xyz The client MUSTauthenticate with theignore unrecognized response parameters. The authorization code string size is left undefined by this specification. The client should avoid making assumptions about code value sizes. The authorization serveras described in Section 3.2.1. For example, the client makesSHOULD document thefollowing HTTP request using transport-layer security (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=client_credentialssize of any value it issues. The authorization server MUSTauthenticate the client. 4.2.3. Access Token Response Ifassociate theaccess token request is valid"code_challenge" andauthorized,"code_challenge_method" values with the issued authorizationserver issues an access token as described in Section 5.1. A refresh token SHOULD NOT be included. Ifcode so therequest failed client authentication or is invalid,code challenge can be verified later. The exact method that theauthorizationserverreturns an error response as described in Section 5.2. An example successful response: HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-store Pragma: no-cache { "access_token": "2YotnFZFEjr1zCsicMWpAA", "token_type": "Bearer", "expires_in": 3600, "example_parameter": "example_value" } 4.3. Extension Grants The clientusesan extension grant type by specifying the grant type using an absolute URI (defined by the authorization server) asto associate thevalue of"code_challenge" with the"grant_type" parameterissued code is out of scope for this specification. The code challenge could be stored on thetoken endpoint,server andby adding any additional parameters necessary. For example, to request an access token using the Device Authorization Grant as defined by [RFC8628] afterassociated with theuser has authorizedcode there. The "code_challenge" and "code_challenge_method" values may be stored in encrypted form in theclient on a separate device,code itself, but theclient makesserver MUST NOT include thefollowing HTTP request using TLS (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code &device_code=GmRhmhcxhwEzkoEqiMEg_DnyEysNkuNhszIySk9eS &client_id=C409020731"code_challenge" value in a response parameter in a form that entities other than the AS can extract. 4.1.2.1. Error Response If theaccess tokenrequest fails due to a missing, invalid, or mismatching redirect URI, or if the client identifier isvalid and authorized,missing or invalid, the authorization serverissues an access tokenSHOULD inform the resource owner of the error andoptional refresh token as describedMUST NOT automatically redirect the user agent to the invalid redirect URI. An AS MUST reject requests without a "code_challenge" from public clients, and MUST reject such requests from other clients unless there is reasonable assurance that the client mitigates authorization code injection in other ways. See Section5.1.7.8 for details. If therequest failed client authentication or is invalid,server does not support the requested "code_challenge_method" transformation, the authorization endpoint MUST return the authorizationserver returns anerror responseas described in Section 5.2. 5. Issuing an Access Tokenwith "error" value set to "invalid_request". The "error_description" or the response of "error_uri" SHOULD explain the nature of error, e.g., transform algorithm not supported. If the resource owner denies the accesstokenrequestis valid and authorized,or if the request fails for reasons other than a missing or invalid redirect URI, the authorization serverissues an access token and optional refresh token as described in Section 5.1. Ifinforms therequest failedclientauthenticationby adding the following parameters to the query component of the redirect URI using the "application/x-www-form-urlencoded" format, per Appendix B: "error": REQUIRED. A single ASCII [USASCII] error code from the following: "invalid_request": The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or isinvalid,otherwise malformed. "unauthorized_client": The client is not authorized to request an authorization code using this method. "access_denied": The resource owner or authorization server denied the request. "unsupported_response_type": The authorization serverreturnsdoes not support obtaining anerror response as described in Section 5.2. 5.1. Successful Responseauthorization code using this method. "invalid_scope": The requested scope is invalid, unknown, or malformed. "server_error": The authorization serverissues an access token and optional refresh token by creatingencountered anHTTP response body usingunexpected condition that prevented it from fulfilling the"application/json" media type as defined by [RFC8259] withrequest. (This error code is needed because a 500 Internal Server Error HTTP status code cannot be returned to thefollowing parameters andclient via an HTTP200 (OK) status code: "access_token": REQUIRED.redirect.) "temporarily_unavailable": Theaccess token issued by theauthorizationserver. "token_type": REQUIRED. The type of the access token issued as described in Section 7.1. Valueserver iscase insensitive. "expires_in": RECOMMENDED. The lifetime in secondscurrently unable to handle the request due to a temporary overloading or maintenance of theaccess token. For example,server. (This error code is needed because a 503 Service Unavailable HTTP status code cannot be returned to thevalue "3600" denotes thatclient via an HTTP redirect.) Values for theaccess token will expire in one hour from"error" parameter MUST NOT include characters outside thetimeset %x20-21 / %x23-5B / %x5D-7E. "error_description": OPTIONAL. Human-readable ASCII [USASCII] text providing additional information, used to assist theresponse was generated. If omitted,client developer in understanding theauthorization server SHOULD provideerror that occurred. Values for theexpiration time via other means or document"error_description" parameter MUST NOT include characters outside thedefault value. "refresh_token":set %x20-21 / %x23-5B / %x5D-7E. "error_uri": OPTIONAL.The refresh token, which can be used to obtain new access tokens usingA URI identifying a human-readable web page with information about thesame authorization grant as described in Section 6. "scope": OPTIONAL, if identicalerror, used to provide thescope requested byclient developer with additional information about theclient; otherwise, REQUIRED. The scope oferror. Values for theaccess token as described by Section 3.3. The parameters are serialized into a JavaScript Object Notation (JSON) structure by adding each"error_uri" parameteratMUST conform to thehighest structure level. Parameter names and string values are included as JSON strings. Numerical values are included as JSON numbers. The order of parameters does not matterURI-reference syntax andcan vary. The authorization serverthus MUST NOT include characters outside theHTTP "Cache-Control" response header field [RFC7234] withset %x21 / %x23-5B / %x5D-7E. "state": REQUIRED if avalue of "no-store""state" parameter was present inany response containing tokens, credentials, or other sensitive information, as well asthe"Pragma" response header field [RFC7234] with a value of "no-cache". For example: HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-store Pragma: no-cache { "access_token":"2YotnFZFEjr1zCsicMWpAA", "token_type":"Bearer", "expires_in":3600, "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA", "example_parameter":"example_value" } TheclientMUST ignore unrecognized value names in the response.authorization request. Thesizes of tokens and other valuesexact value received from the client. For example, the authorization serverare left undefined. The client should avoid making assumptions about value sizes. The authorization server SHOULD documentredirects thesize of any value it issues. 5.2. Error Responseuser agent by sending the following HTTP response: HTTP/1.1 302 Found Location: https://client.example.com/cb?error=access_denied&state=xyz 4.1.3. Token Endpoint Extension The authorizationserver respondsgrant type is identified at the token endpoint withan HTTP 400 (Bad Request) status code (unless specified otherwise) and includesthe "grant_type" value of "authorization_code". If this value is set, the following additional token request parameterswith the response: "error":beyond Section 3.2.2 are required: "code": REQUIRED.A single ASCII [USASCII] errorThe authorization code received from thefollowing: "invalid_request": The request is missing a required parameter, includes an unsupportedauthorization server. "redirect_uri": REQUIRED, if the "redirect_uri" parametervalue (other than grant type), repeats a parameter, includes multiple credentials, utilizes more than one mechanism for authenticatingwas included in the authorization request as described in Section 4.1.1, and their values MUST be identical. "code_verifier": REQUIRED, if theclient, contains a "code_verifier" although no"code_challenge" parameter wassentincluded in the authorizationrequest, or is otherwise malformed. "invalid_client": Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method).request. MUST NOT be used otherwise. Theauthorization server MAY return an HTTP 401 (Unauthorized) statusoriginal codeto indicate which HTTP authentication schemes are supported. Ifverifier string. For example, the clientattempted to authenticate via the "Authorization" request header field,makes theauthorization server MUST respond with anfollowing HTTP401 (Unauthorized) status code and include the "WWW-Authenticate" response header field matching the authentication scheme used by the client. "invalid_grant": The provided authorization grant (e.g., authorization code, resource owner credentials) or refresh token is invalid, expired, revoked, does not matchrequest using TLS (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bed In addition to theredirect URI usedprocessing rules in Section 3.2.2, the authorizationrequest, orserver MUST: * ensure that the authorization code was issued toanother client. "unauthorized_client": Thethe authenticated confidential or credentialed client, or if the client isnot authorizedpublic, ensure that the code was issued touse this authorization grant type. "unsupported_grant_type": The"client_id" in the request, * verify that the authorizationgrant typecode isnot supported byvalid, * verify that the "code_verifier" parameter is present if and only if a "code_challenge" parameter was present in the authorizationserver. "invalid_scope": The requested scoperequest, * if a "code_verifier" isinvalid, unknown, malformed, or exceedspresent, verify thescope granted"code_verifier" by calculating theresource owner. Values forcode challenge from the"error" parameter MUST NOT include characters outsidereceived "code_verifier" and comparing it with theset %x20-21 / %x23-5B / %x5D-7E. "error_description": OPTIONAL. Human-readable ASCII [USASCII] text providing additional information, usedpreviously associated "code_challenge", after first transforming it according toassisttheclient developer in understanding"code_challenge_method" method specified by theerrorclient, and * ensure thatoccurred. Values forthe"error_description""redirect_uri" parameterMUST NOT include characters outside the set %x20-21 / %x23-5B / %x5D-7E. "error_uri": OPTIONAL. A URI identifying a human-readable web page with information about the error, used to provide the client developer with additional information about the error. Values foris present if the"error_uri""redirect_uri" parameterMUST conform to the URI-reference syntax and thus MUST NOT include characters outside the set %x21 / %x23-5B / %x5D-7E. The parameters arewas includedin the payload of the HTTP response using the "application/json" media type as defined by [RFC7159]. The parameters are serialized into a JSON structure by adding each parameter atin thehighest structure level. Parameter namesinitial authorization request as described in Section 4.1.1, andstring values areif includedas JSON strings. Numericalensure that their values areincluded as JSON numbers.identical. 4.2. Client Credentials Grant Theorder of parameters does not matter andclient canvary. For example: HTTP/1.1 400 Bad Request Content-Type: application/json Cache-Control: no-store Pragma: no-cache { "error":"invalid_request" } 6. Refreshingrequest anAccess Token Authorization servers SHOULD determine, based on a risk assessment and their own policies, whether to issue refresh tokensaccess token using only its client credentials (or other supported means of authentication) when the client is requesting access toa certain client. Ifthe protected resources under its control, or those of another resource owner that have been previously arranged with the authorization serverdecides not to issue refresh tokens,(the method of which is beyond the scope of this specification). The clientMAY obtain new access tokenscredentials grant type MUST only be used bystartingconfidential or credentialed clients. +---------+ +---------------+ | | | | | |>--(1)- Client Authentication --->| Authorization | | Client | | Server | | |<--(2)---- Access Token ---------<| | | | | | +---------+ +---------------+ Figure 4: Client Credentials Grant The use of theOAuth flow over, for example initiating a new authorization code request. In such a case,client credentials grant illustrated in Figure 4 includes the following steps: (1) The client authenticates with the authorization servermay utilize cookiesandpersistent grants to optimizerequests an access token from theuser experience. If refresh tokens are issued, those refresh tokens MUST be bound totoken endpoint. (2) The authorization server authenticates thescopeclient, andresource servers as consented byif valid, issues an access token. 4.2.1. Token Endpoint Extension The authorization grant type is identified at theresource owner. Thistoken endpoint with the "grant_type" value of "client_credentials". If this value isto prevent privilege escalation byset, no additional parameters beyond Section 3.2.2 are required/supported: For example, thelegitimateclientand reducemakes theimpact of refresh token leakage. 6.1.following HTTP request using transport-layer security (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=client_credentials The authorization server MUST authenticate the client. 4.3. Refresh TokenRequest To use aGrant The refresh token is a credential issued by the authorization server toobtaina client, which can be used to obtain new (fresh) accesstoken, thetokens based on an existing grant. The clientmakes a request touses this option either because the previous access tokenendpoint by addinghas expired or thefollowing parameters usingclient previously obtained an access token with a scope more narrow than approved by the"application/x-www-form-urlencoded" format (per Appendix B)respective grant and later requires an access token with acharacter encoding of UTF-8 indifferent scope under theHTTP request payload: "grant_type": REQUIRED. Value MUST be set to "refresh_token". "refresh_token": REQUIRED.same grant. 4.3.1. Token Endpoint Extension Therefreshauthorization grant type is identified at the tokenissued toendpoint with theclient. "scope": OPTIONAL. The scope"grant_type" value of "refresh_token". If this value is set, theaccess request as described byfollowing additional parameters beyond Section3.3. The requested scope MUST NOT include any scope not originally granted by the resource owner, and if omitted is treated as equal3.2.2 are required/supported: "refresh_token": REQUIRED. The refresh token issued to thescope originally granted by the resource owner.client. Because refresh tokens are typically long-lasting credentials used to request additional access tokens, the refresh token is bound to the client to which it was issued. Confidential or credentialed clients MUST authenticate with the authorization server as described in Section 3.2.1. For example, the client makes the following HTTP request using transport-layer security (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW Content-Type: application/x-www-form-urlencoded grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIATheIn addition to the processing rules in Section 3.2.2, the authorization server MUST: *require client authentication for confidential or credentialed clients * authenticate the client if client authentication is included and ensure that the refresh token was issued to the authenticated client, and *validate the refresh token. Authorization servers SHOULD utilize one of these methods to detect refresh token replay by malicious actors for public clients: * _Sender-constrained refresh tokens:_ the authorization server cryptographically binds the refresh token to a certain client instance by utilizing [I-D.ietf-oauth-token-binding], [RFC8705], [I-D.ietf-oauth-dpop], or another suitable method. * _Refresh token rotation:_ the authorization server issues a new refresh token with every access token refresh response. The previous refresh token is invalidated but information about the relationship is retained by the authorization server. If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach. The authorization server cannot determine which party submitted the invalid refresh token, but it will revoke the active refresh token. This stops the attack at the cost of forcing the legitimate client to obtain a fresh authorization grant. Implementation note: the grant to which a refresh token belongs may be encoded into the refresh token itself. This can enable an authorization server to efficiently determine the grant to which a refresh token belongs, and by extension, all refresh tokens that need to be revoked. Authorization servers MUST ensure the integrity of the refresh token value in this case, for example, using signatures.6.2.4.3.2. Refresh Token Response If valid and authorized, the authorization server issues an access token as described in Section5.1. If the request failed verification or is invalid, the authorization server returns an error response as described in Section 5.2.3.2.3. The authorization server MAY issue a new refresh token, in which case the client MUST discard the old refresh token and replace it with the new refresh token. The authorization server MAY revoke the old refresh token after issuing a new refresh token to the client. If a new refresh token is issued, the refresh token scope MUST be identical to that of the refresh token included by the client in the request. Authorization servers MAY revoke refresh tokens automatically in case of a security event, such as: * password change * logout at the authorization server Refresh tokens SHOULD expire if the client has been inactive for some time, i.e., the refresh token has not been used to obtain new access tokens for some time. The expiration time is at the discretion of the authorization server. It might be a global value or determined based on the client policy or the grant associated with the refresh token (and its sensitivity).7.4.4. Extension Grants The client uses an extension grant type by specifying the grant type using an absolute URI (defined by the authorization server) as the value of the "grant_type" parameter of the token endpoint, and by adding any additional parameters necessary. For example, to request an access token using the Device Authorization Grant as defined by [RFC8628] after the user has authorized the client on a separate device, the client makes the following HTTP request using TLS (with extra line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code &device_code=GmRhmhcxhwEzkoEqiMEg_DnyEysNkuNhszIySk9eS &client_id=C409020731 If the access token request is valid and authorized, the authorization server issues an access token and optional refresh token as described in Section 3.2.3. If the request failed client authentication or is invalid, the authorization server returns an error response as described in Section 3.2.3.1. 5. Accessing Protected Resources The client accesses protected resources by presenting the access token to the resource server. The resource server MUST validate the access token and ensure that it has not expired and that its scope covers the requested resource. The methods used by the resource server to validate the access token (as well as any error responses) are beyond the scope of this specification, but generally involve an interaction or coordination between the resource server and the authorizationserver, such as usingserver. For example, when the resource server and authorization server are colocated or are part of the same system, they may share a database or other storage; when the two components are operated independently, they may use Token Introspection [RFC7662] or a structured access token format such as a JWT [I-D.ietf-oauth-access-token-jwt]. The method in which the client utilizes the access token toauthenticate withaccess protected resources at the resource server depends on the type of access token issued by the authorization server. Typically, it involves using the HTTP "Authorization" request header field[RFC2617][RFC7235] with an authentication scheme defined by the specification of the access token type used, such as "Bearer", defined below.7.1.5.1. Access Token Types The access token type provides the client with the information required to successfully utilize the access token to make a protected resource request (along with type-specific attributes). The client MUST NOT use an access token if it does not understand the token type. For example, the "Bearer" token type defined in this specification is utilized by simply including the access token string in the request: GET /resource/1 HTTP/1.1 Host: example.com Authorization: Bearer mF_9.B5f-4.1JqM The above example is provided for illustration purposes only. Each access token type definition specifies the additional attributes (if any) sent to the client together with the "access_token" response parameter. It also defines the HTTP authentication method used to include the access token when making a protected resource request.7.2.5.2. Bearer Tokens A Bearer Token is a security token with the property that any party in possession of the token (a "bearer") can use the token in any way that any other party in possession of it can. Using a bearer token does not require a bearer to prove possession of cryptographic key material (proof-of-possession). Bearer tokens may be extended to include proof-of-possession techniques by other specifications.7.2.1.5.2.1. Authenticated Requests This section defines two methods of sending Bearer tokens in resource requests to resource servers. Clients MUST NOT use more than one method to transmit the token in each request.7.2.1.1.5.2.1.1. Authorization Request Header Field When sending the access token in the "Authorization" request header field defined by HTTP/1.1[RFC2617],[RFC7235], the client uses the "Bearer" authentication scheme to transmit the access token. For example: GET /resource HTTP/1.1 Host: server.example.com Authorization: Bearer mF_9.B5f-4.1JqM The syntax of the "Authorization" header field for this scheme follows the usage of the Basic scheme defined in Section 2 of [RFC2617]. Note that, as with Basic, it does not conform to the generic syntax defined in Section 1.2 of [RFC2617] but is compatible with the general authentication framework in HTTP 1.1 Authentication [RFC7235], although it does not follow the preferred practice outlined therein in order to reflect existing deployments. The syntax for Bearer credentials is as follows: b64token = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" ) *"=" credentials = "Bearer" 1*SP b64token Clients SHOULD make authenticated requests with a bearer token using the "Authorization" request header field with the "Bearer" HTTP authorization scheme. Resource servers MUST support this method.7.2.1.2.5.2.1.2. Form-Encoded Body Parameter When sending the access token in the HTTP request payload, the client adds the access token to the request-body using the "access_token" parameter. The client MUST NOT use this method unless all of the following conditions are met: * The HTTP request entity-header includes the "Content-Type" header field set to "application/x-www-form-urlencoded". * The payload follows the encoding requirements of the "application/ x-www-form-urlencoded" content-type as defined by HTML 4.01 [W3C.REC-html401-19991224]. * The HTTP request payload is single-part. * The content to be encoded in the payload MUST consist entirely of ASCII [USASCII] characters. * The HTTP request method is one for which the request-body has defined semantics. In particular, this means that the "GET" method MUST NOT be used. The payload MAY include other request-specific parameters, in which case the "access_token" parameter MUST be properly separated from the request-specific parameters using "&" character(s) (ASCII code 38). For example, the client makes the following HTTP request using transport-layer security: POST /resource HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded access_token=mF_9.B5f-4.1JqM The "application/x-www-form-urlencoded" method SHOULD NOT be used except in application contexts where participating clients do not have access to the "Authorization" request header field. Resource servers MAY support this method.7.2.2.5.2.2. The WWW-Authenticate Response Header Field If the protected resource request does not include authentication credentials or does not contain an access token that enables access to the protected resource, the resource server MUST include the HTTP "WWW-Authenticate" response header field; it MAY include it in response to other conditions as well. The "WWW-Authenticate" header field uses the framework defined by HTTP/1.1[RFC2617].[RFC7235]. All challengesdefined byfor thisspecificationtoken type MUST use the auth-scheme value "Bearer". This scheme MUST be followed by one or moreauth- paramauth-param values. The auth-param attributes used or defined by this specification for this token type are as follows. Other auth-param attributes MAY be used as well. A "realm" attribute MAY be included to indicate the scope of protection in the manner described in HTTP/1.1[RFC2617].[RFC7235]. The "realm" attribute MUST NOT appear more than once. The "scope" attribute is defined in Section3.3.3.2.2.1. The "scope" attribute is a space-delimited list of case-sensitive scope values indicating the required scope of the access token for accessing the requested resource. "scope" values are implementation defined; there is no centralized registry for them; allowed values are defined by the authorization server. The order of "scope" values is not significant. In some cases, the "scope" value will be used when requesting a new access token with sufficient scope of access to utilize the protected resource. Use of the "scope" attribute is OPTIONAL. The "scope" attribute MUST NOT appear more than once. The "scope" value is intended for programmatic use and is not meant to be displayed to end-users. Two example scope values follow; these are taken from the OpenID Connect [OpenID.Messages] and the Open Authentication Technology Committee (OATC) Online Multimedia Authorization Protocol [OMAP] OAuth 2.0 use cases, respectively: scope="openid profile email" scope="urn:example:channel=HBO&urn:example:rating=G,PG-13" If the protected resource request included an access token and failed authentication, the resource server SHOULD include the "error" attribute to provide the client with the reason why the access request was declined. The parameter value is described in Section7.2.3.5.2.3. In addition, the resource server MAY include the "error_description" attribute to provide developers a human-readable explanation that is not meant to be displayed to end-users. It also MAY include the "error_uri" attribute with an absolute URI identifying a human-readable web page explaining the error. The "error", "error_description", and "error_uri" attributes MUST NOT appear more than once. Values for the "scope" attribute (specified in Appendix A.4) MUST NOT include characters outside the set %x21 / %x23-5B / %x5D-7E for representing scope values and %x20 for delimiters between scope values. Values for the "error" and "error_description" attributes (specified in Appendixes A.7 and A.8) MUST NOT include characters outside the set %x20-21 / %x23-5B / %x5D-7E. Values for the "error_uri" attribute (specified in Appendix A.9 of) MUST conform to the URI-reference syntax and thus MUST NOT include characters outside the set %x21 / %x23-5B / %x5D-7E. For example, in response to a protected resource request without authentication: HTTP/1.1 401 Unauthorized WWW-Authenticate: Bearer realm="example" And in response to a protected resource request with an authentication attempt using an expired access token: HTTP/1.1 401 Unauthorized WWW-Authenticate: Bearer realm="example", error="invalid_token", error_description="The access token expired"7.2.3.5.2.3. Error Codes When a request fails, the resource server responds using the appropriate HTTP status code (typically, 400, 401, 403, or 405) and includes one of the following error codes in the response: "invalid_request": The request is missing a required parameter, includes an unsupported parameter or parameter value, repeats the same parameter, uses more than one method for including an access token, or is otherwise malformed. The resource server SHOULD respond with the HTTP 400 (Bad Request) status code. "invalid_token": The access token provided is expired, revoked, malformed, or invalid for other reasons. The resource SHOULD respond with the HTTP 401 (Unauthorized) status code. The client MAY request a new access token and retry the protected resource request. "insufficient_scope": The request requires higher privileges than provided by the access token. The resource server SHOULD respond with the HTTP 403 (Forbidden) status code and MAY include the "scope" attribute with the scope necessary to access the protected resource. If the request lacks any authentication information (e.g., the client was unaware that authentication is necessary or attempted using an unsupported authentication method), the resource server SHOULD NOT include an error code or other error information. For example: HTTP/1.1 401 Unauthorized WWW-Authenticate: Bearer realm="example"7.3.5.3. Error Response If a resource access request fails, the resource server SHOULD inform the client of the error. The method by which the resource server does this is determined by the particular token type, such as the description of Bearer tokens in Section7.2.3. 7.3.1.5.2.3. 5.3.1. Extension Token Types [RFC6750] establishes a common registry in Section 11.4 (https://tools.ietf.org/html/rfc6749#section-11.4) for error values to be shared among OAuth token authentication schemes. New authentication schemes designed primarily for OAuth token authentication SHOULD define a mechanism for providing an error status code to the client, in which the error values allowed are registered in the error registry established by this specification. Such schemes MAY limit the set of valid error codes to a subset of the registered values. If the error code is returned using a named parameter, the parameter name SHOULD be "error". Other schemes capable of being used for OAuth token authentication, but not primarily designed for that purpose, MAY bind their error values to the registry in the same manner. New authentication schemes MAY choose to also specify the use of the "error_description" and "error_uri" parameters to return error information in a manner parallel to their usage in this specification.7.4.6. Extensibility 6.1. Defining Access TokenSecurity Considerations 7.4.1. Security Threats The following list presents several common threats against protocols utilizing some form of tokens. This list of threats is based on NIST Special Publication 800-63 [NIST800-63]. 7.4.1.1. Token manufacture/modification An attacker may generate a bogus token or modify the token contents (such as the authentication or attribute statements) of an existing token, causing the resource server to grant inappropriate access to the client. For example, an attacker may modify theTypes Access tokento extend the validity period; a malicious client may modify the assertion to gain access to information that they should nottypes can beable to view. 7.4.1.2. Token disclosure Tokens may contain authentication and attribute statements that include sensitive information. 7.4.1.3. Token redirect An attacker uses a token generated for consumption bydefined in oneresource server to gain access to a different resource server that mistakenly believes the token to be for it. 7.4.1.4. Token replay An attacker attempts to use a token that has already been used with that resource serverof two ways: registered in thepast. 7.4.2. Threat Mitigation A large range of threats can be mitigated by protectingAccess Token Types registry (following thecontentsprocedures in Section 11.1 ofthe token by using a digital signature. Alternatively, a bearer token can contain a reference to authorization information, rather than encoding the information directly. Such references MUST be infeasible for an attacker to guess;[RFC6749]), or by using areference may require an extra interaction betweenunique absolute URI as its name. Types utilizing aserver and the token issuer to resolve the referenceURI name SHOULD be limited tothe authorization information. The mechanics of such an interactionvendor-specific implementations that are notdefined by this specification. This document does not specify the encoding orcommonly applicable, and are specific to thecontentsimplementation details of thetoken; hence, detailed recommendations about the means of guaranteeing token integrity protectionresource server where they areoutside the scope of this document. The token integrity protectionused. All other types MUST besufficientregistered. Type names MUST conform topreventthe type-name ABNF. If the type definition includes a new HTTP authentication scheme, the type name SHOULD be identical to the HTTP authentication scheme name (as defined by [RFC2617]). The tokenfrom being modified. To deal with token redirect, ittype "example" isimportantreserved for use in examples. type-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA 6.2. Defining New Endpoint Parameters New request or response parameters for use with the authorizationserver to includeendpoint or theidentitytoken endpoint are defined and registered in the OAuth Parameters registry following the procedure in Section 11.2 of [RFC6749]. Parameter names MUST conform to theintended recipients (the audience), typically a single resource server (orparam-name ABNF, and parameter values syntax MUST be well-defined (e.g., using ABNF, or alist of resource servers), inreference to thetoken. Restrictingsyntax of an existing parameter). param-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA Unregistered vendor-specific parameter extensions that are not commonly applicable and that are specific to theuseimplementation details of thetoken to a specific scope is also RECOMMENDED. Theauthorization serverMUST implement TLS. Which version(s) oughtwhere they are used SHOULD utilize a vendor-specific prefix that is not likely to conflict with other registered values (e.g., begin with 'companyname_'). 6.3. Defining New Authorization Grant Types New authorization grant types can beimplemented will vary over time and will depend ondefined by assigning them a unique absolute URI for use with thewidespread deployment and known security vulnerabilities at"grant_type" parameter. If thetime of implementation. To protect againstextension grant type requires additional tokendisclosure, confidentiality protectionendpoint parameters, they MUST beapplied using TLS with a ciphersuite that provides confidentiality and integrity protection. This requires that the communication interaction between the client andregistered in theauthorization server, as wellOAuth Parameters registry as described by Section 11.2 of [RFC6749]. 6.4. Defining New Authorization Endpoint Response Types New response types for use with theinteraction between the clientauthorization endpoint are defined and registered in theresource server, utilize confidentiality and integrity protection. Since TLS is mandatory to implement andAuthorization Endpoint Response Types registry following the procedure in Section 11.3 of [RFC6749]. Response type names MUST conform touse with this specification,the response-type ABNF. response-type = response-name *( SP response-name ) response-name = 1*response-char response-char = "_" / DIGIT / ALPHA If a response type contains one or more space characters (%x20), it is compared as a space-delimited list of values in which thepreferred approach for preventing token disclosure via the communication channel. For those cases where the client is prevented from observingorder of values does not matter. Only one order of values can be registered, which covers all other arrangements of thecontentssame set of values. For example, an extension can define and register thetoken, token encryption MUST"code other_token" response type. Once registered, the same combination cannot beapplied in additionregistered as "other_token code", but both values can be used to denote theusage of TLS protection. As a further defense againstsame response type. 6.5. Defining Additional Error Codes In cases where protocol extensions (i.e., access tokendisclosure, the client MUST validate the TLS certificate chain when making requeststypes, extension parameters, or extension grant types) require additional error codes toprotected resources, including checkingbe used with theCertificate Revocation List (CRL) [RFC5280]. Cookies are typically transmitted inauthorization code grant error response (Section 4.1.2.1), theclear. Thus, any information contained in them is at risk of disclosure. Therefore, Bearer tokens MUST NOTtoken error response (Section 3.2.3.1), or the resource access error response (Section 5.3), such error codes MAY bestored in cookies that candefined. Extension error codes MUST besent inregistered (following theclear, as any informationprocedures inthem is at riskSection 11.4 ofdisclosure. See "HTTP State Management Mechanism" [RFC6265] for security considerations about cookies. In some deployments, including those utilizing load balancers, the TLS connection to[RFC6749]) if theresource server terminates priorextension they are used in conjunction with is a registered access token type, a registered endpoint parameter, or an extension grant type. Error codes used with unregistered extensions MAY be registered. Error codes MUST conform to theactual server that provides the resource. This could leave the token unprotected between the front-end server where the TLS connection terminateserror ABNF andthe back-end server that provides the resource. In such deployments, sufficient measures MUSTSHOULD beemployedprefixed by an identifying name when possible. For example, an error identifying an invalid value set toensure confidentiality ofthetoken betweenextension parameter "example" SHOULD be named "example_invalid". error = 1*error-char error-char = %x20-21 / %x23-5B / %x5D-7E 7. Security Considerations As a flexible and extensible framework, OAuth's security considerations depend on many factors. The following sections provide implementers with security guidelines focused on thefront-endthree client profiles described in Section 2.1: web application, browser- based application, andback-end servers; encryption ofnative application. A comprehensive OAuth security model and analysis, as well as background for thetokenprotocol design, isone such possible measure. To deal with token captureprovided by [RFC6819] andreplay, the[I-D.ietf-oauth-security-topics]. 7.1. Access Token Security Considerations 7.1.1. Security Threats The followingrecommendations are made: First, the lifetimelist presents several common threats against protocols utilizing some form ofthe token MUST be limited; one meanstokens. This list ofachieving thisthreats isby puttingbased on NIST Special Publication 800-63 [NIST800-63]. 7.1.1.1. Access token manufacture/modification An attacker may generate avalidity time field inside the protected part of the token. Note that using short-lived (one hourbogus access token orless) tokens reduces the impact of them being leaked. Second, confidentiality protection of the exchanges between the client andmodify theauthorization server and betweentoken contents (such as theclient andauthentication or attribute statements) of an existing token, causing the resource serverMUST be applied. As a consequence, no eavesdropper along the communication path is abletoobservegrant inappropriate access to thetoken exchange. Consequently, suchclient. For example, anon-path adversary cannot replay the token. Furthermore, when presentingattacker may modify the token toa resource server, the client MUST verify the identity of that resource server, as per Section 3.1 of "HTTP Over TLS" [RFC2818]. Note thatextend the validity period; a malicious clientMUST validate the TLS certificate chain when making these requests to protected resources. Presenting the token to an unauthenticated and unauthorized resource server or failing to validatemay modify thecertificate chain will allow adversariesassertion tosteal the token andgainunauthorizedaccess toprotected resources. 7.4.3. Summary of Recommendations 7.4.3.1. Safeguard bearer tokens Client implementations MUST ensureinformation thatbearer tokens are not leaked to unintended parties, astheywillshould not be able touse them to gain access to protected resources. This is the primary security consideration when using bearerview. 7.1.1.2. Access token disclosure Access tokens may contain authentication andunderlies all the more specific recommendationsattribute statements thatfollow. 7.4.3.2. Validate TLS certificate chains The client MUST validate the TLS certificate chain when making requests to protected resources. Failinginclude sensitive information. 7.1.1.3. Access token redirect An attacker uses an access token generated for consumption by one resource server todo so may enable DNS hijacking attacksgain access tosteala different resource server that mistakenly believes the tokenand gain unintended access. 7.4.3.3. Always use TLS (https) Clients MUST always use TLS (https) or equivalent transport security when making requests with bearer tokens. Failingtodo so exposes thebe for it. 7.1.1.4. Access token replay An attacker attempts tonumerous attacksuse an access token thatcould give attackers unintended access. 7.4.3.4. Don't store bearer tokens in HTTP cookies Implementations MUST NOT store bearer tokens within cookieshas already been used with that resource server in the past. 7.1.2. Threat Mitigation A large range of threats can besent inmitigated by protecting theclear (which iscontents of thedefault transmission mode for cookies). Implementations that do storeaccess token by using a digital signature. Alternatively, a bearertokens in cookiestoken can contain a reference to authorization information, rather than encoding the information directly. Such references MUSTtake precautions against cross-site request forgery. 7.4.3.5. Issue short-lived bearer tokens Authorization servers SHOULD issue short-lived (one hour or less) bearer tokens, particularly when issuing tokensbe infeasible for an attacker toclients that run withinguess; using aweb browser or other environments where information leakagereference mayoccur. Using short-lived bearer tokens can reducerequire an extra interaction between a server and theimpactaccess token issuer to resolve the reference to the authorization information. The mechanics ofthemsuch an interaction are not defined by this specification. This document does not specify the encoding or the contents of the access token; hence, detailed recommendations about the means of guaranteeing access token integrity protection are outside the scope of this specification. The access token integrity protection MUST be sufficient to prevent the token from beingleaked. 7.4.3.6. Issue scoped bearer tokens Authorization servers SHOULD issue bearer tokens that containmodified. One example of anaudience restriction, scoping their useencoding and signing mechanism for access tokens is described in [I-D.ietf-oauth-access-token-jwt]. To deal with access token redirects, it is important for the authorization server to include the identity of the intendedrelying party or setrecipients (the audience), typically a single resource server (or a list ofrelying parties. 7.4.3.7. Don't pass bearer tokensresource servers), inpage URLs Bearer tokensthe token. Restricting the use of the token to a specific scope is also RECOMMENDED. The authorization server MUSTNOT be passed in page URLs (for example,implement TLS asquery string parameters). Instead, bearer tokens SHOULD be passeddescribed inHTTP message headers or message bodiesWhich version(s) ought to be implemented will vary over time and will depend on the widespread deployment and known security vulnerabilities at the time of implementation. Refer to Section 1.5.[BCP195] forwhichup to date recommendations on transport layer security. To protect against access token disclosure, confidentiality protection MUST be applied using TLS with a ciphersuite that provides confidentialitymeasures are taken. Browsers, web servers,andother software may not adequately secure URLs inintegrity protection. This requires that thebrowser history, web server logs,communication interaction between the client andother data structures. If bearer tokens are passed in page URLs, attackers might be ablethe authorization server, as well as the interaction between the client and the resource server, utilize confidentiality and integrity protection. Since TLS is mandatory tosteal them fromimplement and to use with this specification, it is thehistory data, logs, or other unsecured locations. 7.4.4. Token Replay Prevention A sender-constrained accesspreferred approach for preventing tokenscopesdisclosure via the communication channel. For those cases where the client is prevented from observing theapplicabilitycontents ofanthe access token, token encryption MUST be applied in addition toa certain sender. This sender is obliged to demonstrate knowledge of a certain secret as prerequisite fortheacceptanceusage ofthat access token at the recipient (e.g.,TLS protection. As aresource server). Authorization and resource servers SHOULD use mechanisms for sender- constrained access tokens to preventfurther defense against tokenreplay as described in Section 4.8.1.1.2 of [I-D.ietf-oauth-security-topics]. The use of Mutualdisclosure, the client MUST validate the TLSfor OAuth 2.0 [RFC8705] is RECOMMENDED. It is RECOMMENDEDcertificate chain when making requests touse end-to-end TLS.protected resources, including checking the Certificate Revocation List (CRL) [RFC5280]. If cookies are transmitted without TLStraffic needs to be terminatedprotection, any information contained in them is atan intermediary, refer to Section 4.11risk of[I-D.ietf-oauth-security-topics] for further security advice. 7.4.5. Access Token Privilege Restriction The privileges associated with an access token SHOULDdisclosure. Therefore, Bearer tokens MUST NOT berestricted tostored in cookies that can be sent in theminimum requiredclear, as any information in them is at risk of disclosure. See "HTTP State Management Mechanism" [RFC6265] forthe particular application or use case. This prevents clients from exceeding the privileges authorized by the resource owner. It also prevents users from exceeding their privileges authorized by the respectivesecuritypolicy. Privilege restrictions also help to reduce the impact of access token leakage.considerations about cookies. Inparticular, access tokens SHOULD be restricted to certain resource servers (audience restriction), preferably to a single resource server. To put this into effect,some deployments, including those utilizing load balancers, theauthorization server associatesTLS connection to theaccess token with certain resource servers and everyresource serveris obligedterminates prior toverify, for every request, whethertheaccess token sent with that request was meant to be used foractual server thatparticular resource server. If not,provides theresource server MUST refuse to serveresource. This could leave therespective request. Clients and authorization servers MAY utilizetoken unprotected between theparameters "scope" or "resource" as specified in this documentfront-end server where the TLS connection terminates and[RFC8707], respectively, to determinetheresourceback-end serverthey want to access. Additionally, access tokens SHOULDthat provides the resource. In such deployments, sufficient measures MUST berestrictedemployed tocertain resources and actions on resource servers or resources. To put this into effect, the authorization server associatesensure confidentiality of the access tokenwithbetween therespective resource and actionsfront-end andevery resource server is obliged to verify, for every request, whetherback- end servers; encryption of theaccesstokensentis one such possible measure. To deal withthat request was meant to be used for that particular action on the particular resource. If not,access token capture and replay, theresource server must refuse to servefollowing recommendations are made: First, therespective request. Clients and authorization servers MAY utilizelifetime of theparameter "scope" and "authorization_details" as specified in [I-D.ietf-oauth-rar] to determine those resources and/or actions. 8. Extensibility 8.1. Defining Access Token Types Accesstokentypes canMUST bedefined inlimited; one means of achieving this is by putting a validity time field inside the protected part of the token. Note that using short- lived tokens reduces the impact of them being leaked. Second, confidentiality protection oftwo ways: registered intheAccess Token Types registry (followingexchanges between theprocedures in Section 11.1 of [RFC6749]), or by using a unique absolute URI as its name. Types utilizing a URI name SHOULD be limited to vendor-specific implementations that are not commonly applicable,client andare specific totheimplementation details ofauthorization server and between the client and the resource serverwhere they are used. All other typesMUST beregistered. Type names MUST conformapplied. As a consequence, no eavesdropper along the communication path is able to observe thetype-name ABNF. Iftoken exchange. Consequently, such an on-path adversary cannot replay thetype definition includestoken. Furthermore, when presenting the token to anew HTTP authentication scheme,resource server, thetype name SHOULD be identicalclient MUST verify the identity of that resource server, as per [BCP195] and Section 3.1 of "HTTP Over TLS" [RFC2818]. Note that the client MUST validate the TLS certificate chain when making these requests to protected resources. Presenting theHTTP authentication scheme name (as defined by [RFC2617]). Thetokentype "example" is reserved for use in examples. type-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA 8.2. Defining New Endpoint Parameters New requestto an unauthenticated and unauthorized resource server orresponse parameters for use withfailing to validate theauthorization endpoint orcertificate chain will allow adversaries to steal the token and gain unauthorized access to protected resources. 7.1.3. Summary of Recommendations 7.1.3.1. Safeguard bearer tokens Client implementations MUST ensure that bearer tokens are not leaked to unintended parties, as they will be able to use them to gain access to protected resources. This is thetoken endpoint are definedprimary security consideration when using bearer tokens andregistered in the OAuth Parameters registry followingunderlies all theprocedure in Section 11.2 of [RFC6749]. Parameter namesmore specific recommendations that follow. 7.1.3.2. Validate TLS certificate chains The client MUSTconformvalidate the TLS certificate chain when making requests to protected resources. Failing to do so may enable DNS hijacking attacks to steal theparam-name ABNF,token andparameter values syntaxgain unintended access. 7.1.3.3. Always use TLS (https) Clients MUSTbe well-defined (e.g., using ABNF,always use TLS (https) ora referenceequivalent transport security when making requests with bearer tokens. Failing to do so exposes thesyntax of an existing parameter). param-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA Unregistered vendor-specific parameter extensionstoken to numerous attacks thatare not commonly applicable andcould give attackers unintended access. 7.1.3.4. Don't store bearer tokens in HTTP cookies Implementations MUST NOT store bearer tokens within cookies thatare specific tocan be sent in theimplementation details ofclear (which is theauthorization server where they are used SHOULD utilize a vendor-specific prefixdefault transmission mode for cookies). Implementations thatis not likelydo store bearer tokens in cookies MUST take precautions against cross-site request forgery. 7.1.3.5. Issue short-lived bearer tokens Authorization servers SHOULD issue short-lived bearer tokens, particularly when issuing tokens toconflict withclients that run within a web browser or otherregistered values (e.g., begin with 'companyname_'). 8.3. Defining New Authorization Grant Types New authorization grant typesenvironments where information leakage may occur. Using short-lived bearer tokens canbe defined by assigningreduce the impact of thema unique absolute URI forbeing leaked. 7.1.3.6. Issue scoped bearer tokens Authorization servers SHOULD issue bearer tokens that contain an audience restriction, scoping their usewith the "grant_type" parameter. Ifto theextension grant type requires additional token endpoint parameters, theyintended relying party or set of relying parties. 7.1.3.7. Don't pass bearer tokens in page URLs Bearer tokens MUST NOT beregisteredpassed inthe OAuth Parameters registrypage URLs (for example, asdescribed by Section 11.2 of [RFC6749]. 8.4. Defining New Authorization Endpoint Response Types New response typesquery string parameters). Instead, bearer tokens SHOULD be passed in HTTP message headers or message bodies foruse with the authorization endpointwhich confidentiality measures aredefinedtaken. Browsers, web servers, andregisteredother software may not adequately secure URLs in theAuthorization Endpoint Response Types registry following the procedurebrowser history, web server logs, and other data structures. If bearer tokens are passed inSection 11.3 of [RFC6749]. Response type names MUST conformpage URLs, attackers might be able to steal them from theresponse-type ABNF. response-type = response-name *( SP response-name ) response-name = 1*response-char response-char = "_" / DIGIT / ALPHA If a response type contains onehistory data, logs, ormore space characters (%x20), it is compared as a space-delimited list of values in whichother unsecured locations. 7.1.4. Token Replay Prevention A sender-constrained access token scopes theorder of values does not matter. Only one orderapplicability ofvalues can be registered, which covers all other arrangementsan access token to a certain sender. This sender is obliged to demonstrate knowledge of a certain secret as prerequisite for thesame setacceptance ofvalues. For example, an extension can define and register the "code other_token" response type. Once registered,that access token at thesame combination cannot be registeredrecipient (e.g., a resource server). Authorization and resource servers SHOULD use mechanisms for sender- constrained access tokens to prevent token replay as"other_token code", but both values candescribed in Section 4.8.1.1.2 of [I-D.ietf-oauth-security-topics]. The use of Mutual TLS for OAuth 2.0 [RFC8705] is RECOMMENDED. It is RECOMMENDED to use end-to-end TLS. If TLS traffic needs to beusedterminated at an intermediary, refer todenote the same response type. 8.5. Defining Additional Error Codes In cases where protocol extensions (i.e.,Section 4.11 of [I-D.ietf-oauth-security-topics] for further security advice. 7.1.5. Access Token Privilege Restriction The privileges associated with an access tokentypes, extension parameters, or extension grant types) require additional error codes toSHOULD beused withrestricted to theauthorization code grant error response (Section 4.1.2.1),minimum required for thetoken error response (Section 5.2),particular application or use case. This prevents clients from exceeding the privileges authorized by the resource owner. It also prevents users from exceeding their privileges authorized by the respective security policy. Privilege restrictions also help to reduce the impact of accesserror response (Section 7.3), such error codes MAY be defined. Extension error codes MUSTtoken leakage. In particular, access tokens SHOULD beregistered (followingrestricted to certain resource servers (audience restriction), preferably to a single resource server. To put this into effect, theprocedures in Section 11.4 of [RFC6749]) ifauthorization server associates theextension they are used in conjunctionaccess token with certain resource servers and every resource server isa registeredobliged to verify, for every request, whether the access tokentype, a registered endpoint parameter, or an extension grant type. Error codes usedsent withunregistered extensions MAYthat request was meant to beregistered. Error codesused for that particular resource server. If not, the resource server MUSTconformrefuse to serve theerror ABNFrespective request. Clients andSHOULD be prefixed by an identifying name when possible. For example, an error identifying an invalid value setauthorization servers MAY utilize the parameters "scope" or "resource" as specified in this document and [RFC8707], respectively, to determine theextension parameter "example"resource server they want to access. Additionally, access tokens SHOULD benamed "example_invalid". error = 1*error-char error-char = %x20-21 / %x23-5B / %x5D-7E 9. Security Considerations As a flexiblerestricted to certain resources andextensible framework, OAuth's security considerations dependactions onmany factors. The following sections provide implementersresource servers or resources. To put this into effect, the authorization server associates the access token withsecurity guidelines focused onthethree client profiles described in Section 2.1: web application, browser- based application, and native application. A comprehensive OAuth security modelrespective resource andanalysis, as well as background for the protocol design, is provided by [RFC6819]actions and[I-D.ietf-oauth-security-topics]. 9.1. Client Authentication Authorization servers SHOULD use client authentication if possible. Itevery resource server isRECOMMENDEDobliged touse asymmetric (public-key based) methodsverify, forclient authentication such as mTLS [RFC8705] or "private_key_jwt" [OpenID]. When asymmetric methodsevery request, whether the access token sent with that request was meant to be used forclient authentication are used,that particular action on the particular resource. If not, the resource server must refuse to serve the respective request. Clients and authorization serversdo not needMAY utilize the parameter "scope" and "authorization_details" as specified in [I-D.ietf-oauth-rar] tostore sensitive symmetric keys, making these methods more robust against a number of attacks. Authorizationdetermine those resources and/or actions. 7.2. Client Authentication The authorization server MUST only rely on client authentication if the process of issuance/registration and distribution of the underlying credentials ensures their confidentiality. When client authentication is not possible, the authorization server SHOULD employ other means to validate the client's identity - for example, by requiring the registration of the client redirect URI or enlisting the resource owner to confirm identity. A valid redirect URI is not sufficient to verify the client's identity when asking for resource owner authorization but can be used to prevent delivering credentials to a counterfeit client after obtaining resource owner authorization. The authorization server must consider the security implications of interacting with unauthenticated clients and take measures to limit the potential exposure of other credentials (e.g., refresh tokens) issued to such clients. The privileges an authorization server associates with a certain client identity MUST depend on the assessment of the overall process for client identification and client credential lifecycle management. For example, authentication of a dynamically registered client just ensures the authorization server it is talking to the same client again. In contrast, if there is a web application whose developer's identity was verified, who signed a contract and is issued a client secret that is only used in a secure backend service, the authorization server might allow this client to access moresensiblesensitive services or to use the clientcredentialcredentials grant type.9.1.1.7.2.1. Client Authentication of Native Apps Secrets that are statically included as part of an app distributed to multiple users should not be treated as confidential secrets, as one user may inspect their copy and learn the shared secret. For this reason, it is NOT RECOMMENDED for authorization servers to require client authentication of public native apps clients using a shared secret, as this serves little value beyond client identification which is already provided by the "client_id" request parameter. Authorization servers that still require a statically included shared secret for native app clients MUST treat the client as a public client (as defined inSection 2.1), and not accept the secret as proof of the client's identity. Without additional measures, such clients are subject to client impersonation (see Section 9.3.1). 9.2. Registration of Native App Clients Except when using a mechanism like Dynamic Client Registration [RFC7591] to provision per-instance secrets, native apps are classified as public clients, as defined in Section 2.1; they MUST be registered with the authorization server as such. Authorization servers MUST record the client type in the client registration details in order to identify and process requests accordingly. Authorization servers MUST require clients to register their complete redirect URI (including the path component) and reject authorization requests that specify a redirect URI that doesn't exactly match the one that was registered; the exception is loopback redirects, where an exact match is required except for the port URI component. For private-use URI scheme-based redirects, authorization servers SHOULD enforce the requirement in Section 10.3.1 that clients use schemes that are reverse domain name based. At a minimum, any private-use URI scheme that doesn't contain a period character (".") SHOULD be rejected. In addition to the collision-resistant properties, requiring a URI scheme based on a domain name that is underSection 2.1), and not accept thecontrolsecret as proof of theapp can helpclient's identity. Without additional measures, such clients are subject toprove ownership in the eventclient impersonation (see Section 7.4.1). 7.3. Registration of Native App Clients Except when using adispute where two apps claim the same private-use URI scheme (where one app is acting maliciously). For example, if twomechanism like Dynamic Client Registration [RFC7591] to provision per-instance secrets, native appsclaimed "com.example.app",are classified as public clients, as defined in Section 2.1; they MUST be registered with theowner of "example.com" could petitionauthorization server as such. Authorization servers MUST record theapp store operator to removeclient type in thecounterfeit app. Such a petition is harderclient registration details in order toprove if a generic URI scheme was used.identify and process requests accordingly. Authorization servers MAY request the inclusion of other platform- specific information, such as the app package or bundle name, or other information that may be useful for verifying the calling app's identity on operating systems that support such functions.9.3. Client Impersonation A malicious client can impersonate another client and obtain access to protected resources if the impersonated client fails to, or is unable to, keep its client credentials confidential. TheFor private-use URI scheme-based redirect URIs, authorizationserver MUST authenticateservers SHOULD require that theclient whenever possible. IfURI scheme be based on a domain name that is under theauthorization server cannot authenticatecontrol of theclient dueapp. In addition to theclient's nature, the authorization server MUST requirecollision-resistant properties, this can help to prove ownership in theregistrationevent ofany redirecta dispute where two apps claim the same private-use URIused for receiving authorization responses and SHOULD utilize other means to protect resource owners from such potentially malicious clients.scheme (where one app is acting maliciously). For example, if two apps claimed "com.example.app", theauthorization server can engage the resourceowner of "example.com" could petition the app store operator toassist in identifyingremove the counterfeit app. Such a petition is harder to prove if a generic URI scheme was used. 7.4. Client Impersonation A malicious client can impersonate another client and obtain access to protected resources if the impersonated client fails to, or is unable to, keep itsorigin.client credentials confidential. The authorization server SHOULD enforce explicit resource owner authentication and provide the resource owner with information about the client and the requested authorization scope and lifetime. It is up to the resource owner to review the information in the context of the current client and to authorize or deny the request. The authorization server SHOULD NOT process repeated authorization requests automatically (without active resource owner interaction) without authenticating the client or relying on other measures to ensure that the repeated request comes from the original client and not an impersonator.9.3.1.7.4.1. Impersonation of Native Apps As stated above, the authorization server SHOULD NOT process authorization requests automatically without user consent or interaction, except when the identity of the client can be assured. This includes the case where the user has previously approved an authorization request for a given clientidID - unless the identity of the client can be proven, the request SHOULD be processed as if no previous request had been approved. Measures such as claimed "https" scheme redirects MAY be accepted by authorization servers as identity proof. Some operating systems may offer alternative platform-specific identity features that MAY be accepted, as appropriate.9.4. Access Tokens Access token credentials (as well as any confidential access token attributes) MUST be kept confidential in transit and storage, and only shared among the authorization server, the resource servers the access token is valid for, and the client to whom the access token is issued. Access token credentials MUST only be transmitted using TLS as described in Section 1.6 with server authentication as defined by [RFC2818]. The authorization server MUST ensure that access tokens cannot be generated, modified, or guessed to produce valid access tokens by unauthorized parties. 9.4.1.7.4.2. Access Token Privilege Restriction The client SHOULD request access tokens with the minimal scope necessary. The authorization server SHOULD take the client identity into account when choosing how to honor the requested scope and MAY issue an access token with less rights than requested. The privileges associated with an access token SHOULD be restricted to the minimum required for the particular application or use case. This prevents clients from exceeding the privileges authorized by the resource owner. It also prevents users from exceeding their privileges authorized by the respective security policy. Privilege restrictions also help to reduce the impact of access token leakage. In particular, access tokens SHOULD be restricted to certain resource servers (audience restriction), preferably to a single resource server. To put this into effect, the authorization server associates the access token with certain resource servers and every resource server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular resource server. If not, the resource server MUST refuse to serve the respective request. Clients and authorization servers MAY utilize the parameters "scope" or "resource" as specified in [RFC8707], respectively, to determine the resource server they want to access.9.4.2.7.4.3. Access Token Replay Prevention Additionally, access tokens SHOULD be restricted to certain resources and actions on resource servers or resources. To put this into effect, the authorization server associates the access token with the respective resource and actions and every resource server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular action on the particular resource. If not, the resource server must refuse to serve the respective request. Clients and authorization servers MAY utilize the parameter "scope" and "authorization_details" as specified in [I-D.ietf-oauth-rar] to determine those resources and/or actions. Authorization and resource servers SHOULD use mechanisms for sender- constrained access tokens to prevent token replay as described in (#pop_tokens). A sender-constrained access token scopes the applicability of an access token to a certain sender. This sender is obliged to demonstrate knowledge of a certain secret as prerequisite for the acceptance of that access token at the recipient (e.g., a resource server). The use of Mutual TLS for OAuth 2.0 [RFC8705] is RECOMMENDED.9.5.7.5. Refresh Tokens Authorization servers MAY issue refresh tokens to clients. Refresh tokens MUST be kept confidential in transit and storage, and shared only among the authorization server and the client to whom the refresh tokens were issued. The authorization server MUST maintain the binding between a refresh token and the client to whom it was issued. Refresh tokens MUST only be transmitted using TLS as described in Section1.61.5 with server authentication as defined by [RFC2818]. The authorization server MUST verify the binding between the refresh token and client identity whenever the client identity can be authenticated. When client authentication is not possible, the authorization server SHOULD issue sender-constrained refresh tokens or use refresh token rotation as described in (#refreshing-an-access- token). The authorization server MUST ensure that refresh tokens cannot be generated, modified, or guessed to produce valid refresh tokens by unauthorized parties.9.6.7.6. Client Impersonating Resource Owner Resource servers may make access control decisions based on the identity of the resource owner as communicated in the "sub" claim returned by the authorization server in a token introspection response [RFC7662] or other mechanisms. If a client is able to choose its own "client_id" during registration with the authorization server, then there is a risk that it can register with the same "sub" value as a privileged user. A subsequent access token obtained under the client credentials grant may be mistaken for an access token authorized by the privileged user if the resource server does not perform additional checks. Authorization servers SHOULD NOT allow clients to influence their "client_id" or "sub" value or any other claim if that can cause confusion with a genuine resource owner. Where this cannot be avoided, authorization servers MUST provide other means for the resource server to distinguish between access tokens authorized by a resource owner from access tokens authorized by the client itself.9.7.7.7. ProtectingRedirect-Based Flowsthe Authorization Code Flow When comparing client redirect URIs against pre-registered URIs, authorization servers MUST utilize exact string matching. This measure contributes to the prevention of leakage of authorization codes and access tokens (see (#insufficient_uri_validation)). It can also help to detect mix-up attacks (see (#mix_up)). Clients MUST NOT expose URLs that forward the user's browser to arbitrary URIs obtained from a query parameter ("open redirector"). Open redirectors can enable exfiltration of authorization codes and access tokens, see (#open_redirector_on_client). Clients MUST prevent Cross-Site Request Forgery (CSRF). In this context, CSRF refers to requests to the redirection endpoint that do not originate at the authorization server, but a malicious third party (see Section 4.4.1.8. of [RFC6819] for details). Clients that have ensured that the authorization server supports the "code_challenge" parameter MAY rely the CSRF protection provided by that mechanism. In OpenID Connect flows, the "nonce" parameter provides CSRF protection. Otherwise, one-time use CSRF tokens carried in the "state" parameter that are securely bound to the user agent MUST be used for CSRF protection (see (#csrf_countermeasures)). In order to prevent mix-up attacks (see (#mix_up)), clients MUST only process redirect responses of the authorization server they sent the respective request to and from the same user agent this authorization request was initiated with. Clients MUST store the authorization server they sent an authorization request to and bind this information to the user agent and check that the authorizationrequestresponse was received from the correct authorization server. Clients MUST ensure that the subsequent access token request, if applicable, is sent to the same authorization server. Clients SHOULD use distinct redirect URIs for each authorization server as a means to identify the authorization server a particular response came from. An AS that redirects a request potentially containing user credentials MUST avoid forwarding these user credentials accidentally (see Section9.7.27.7.2 for details).9.7.1.7.7.1. Loopback Redirect Considerations in Native Apps Loopback interface redirect URIs use the "http" scheme (i.e., without Transport Layer Security (TLS)). This is acceptable for loopback interface redirect URIs as the HTTP request never leaves the device. Clients should open the network port only when starting the authorization request and close it once the response is returned. Clients should listen on the loopback network interface only, in order to avoid interference by other network actors. While redirect URIs using localhost (i.e., "http://localhost:{port}/{path}") function similarly to loopback IP redirects described in Section10.3.3,8.3.3, the use of "localhost" is NOT RECOMMENDED. Specifying a redirect URI with the loopback IP literal rather than "localhost" avoids inadvertently listening on network interfaces other than the loopback interface. It is also less susceptible to client-side firewalls and misconfigured host name resolution on the user's device.9.7.2.7.7.2. HTTP 307 Redirect An AS which redirects a request that potentially contains user credentials MUST NOT use the HTTP 307 status code for redirection. If an HTTP redirection (and not, for example, JavaScript) is used for such a request, AS SHOULD use HTTP status code 303 "See Other". At the authorization endpoint, a typical protocol flow is that the AS prompts the user to enter their credentials in a form that is then submitted (using the HTTP POST method) back to the authorization server. The AS checks the credentials and, if successful, redirects the user agent to the client's redirect URI. If the status code 307 were used for redirection, the user agent would send the user credentials via HTTP POST to the client. This discloses the sensitive credentials to the client. If the relying party is malicious, it can use the credentials to impersonate the user at the AS. The behavior might be unexpected for developers, but is defined in [RFC7231], Section 6.4.7. This status code does not require the user agent to rewrite the POST request to a GET request and thereby drop the form data in the POST request body. In the HTTP standard [RFC7231], only the status code 303 unambigiously enforces rewriting the HTTP POST request to an HTTP GET request. For all other status codes, including the popular 302, user agentscan opt not to rewrite POST to GET requests and therefore to reveal the user credentials to the client. (In practice, however, most user agents will only show this behaviour for 307 redirects.) Therefore, the RECOMMENDED status code for HTTP redirects is 303. 9.8. Authorization Codes The transmission of authorization codes MUST be made over a secure channel, and the client MUST require the use of TLS with its redirect URI if the URI identifies a network resource. Since authorization codes are transmitted via user-agent redirections, they could potentially be disclosed through user-agent historycan opt not to rewrite POST to GET requests and therefore to reveal the user credentials to the client. (In practice, however, most user agents will only show this behaviour for 307 redirects.) Therefore, the RECOMMENDED status code for HTTPreferrer headers.redirects is 303. 7.8. Authorization Codes Authorization codes MUST be short lived and single-use. If the authorization server observes multiple attempts to exchange an authorization code for an access token, the authorization server SHOULD attempt to revoke all refresh and access tokens already granted based on the compromised authorization code. If the client can be authenticated, the authorization servers MUST authenticate the client and ensure that the authorization code was issued to the same client. Clients MUST prevent injection (replay) of authorization codes into the authorization response by attackers. To this end, using "code_challenge" and "code_verifier" is REQUIRED for clients and authorization servers MUST enforce their use, unless both of the following criteria are met: * The client is a confidential client. * In the specific deployment and the specific request, there is reasonable assurance for authorization server that the client implements the OpenID Connect "nonce" mechanism properly. In this case, using and enforcing "code_challenge" and "code_verifier" is still RECOMMENDED. The "code_challenge" or OpenID Connect "nonce" value MUST be transaction-specific and securely bound to the client and the user agent in which the transaction was started. If a transaction leads to an error, fresh values for "code_challenge" or "nonce" MUST be chosen. Historic note: Although PKCE [RFC7636] was originally designed as a mechanism to protect native apps, this advice applies to all kinds of OAuth clients, including web applications and other confidential clients. Clients SHOULD use code challenge methods that do not expose the "code_verifier" in the authorization request. Otherwise, attackers that can read the authorization request (cf. Attacker A4 in (#secmodel)) can break the security provided by this mechanism. Currently, "S256" is the only such method. When an authorization code arrives at the token endpoint, the authorization server MUST do the following check: 1. If there was a "code_challenge" in the authorization request for which this code was issued, there must be a "code_verifier" in the token request, and it MUST be verified according to the steps in Section4.1.3.3.2.2. (This is no change from the current behavior in [RFC7636].) 2. If there was no "code_challenge" in the authorization request, any request to the token endpoint containing a "code_verifier" MUST be rejected. Authorization servers MUST support the "code_challenge" and "code_verifier" parameters. Authorization servers MUST provide a way to detect their support for the "code_challenge" mechanism. To this end, they MUST either (a) publish the element "code_challenge_methods_supported" in their AS metadata ([RFC8414]) containing the supported "code_challenge_method"s (which can be used by the client to detect support) or (b) provide a deployment-specific way to ensure or determine support by the AS.9.9.7.9. Request Confidentiality Access tokens, refresh tokens, authorization codes, and client credentials MUST NOT be transmitted in the clear. The "state" and "scope" parameters SHOULD NOT include sensitive client or resource owner information in plain text, as they can be transmitted over insecure channels or stored insecurely.9.10.7.10. Ensuring Endpoint Authenticity In order to prevent man-in-the-middle attacks, the authorization server MUST require the use of TLS with server authentication as defined by [RFC2818] for any request sent to the authorization and token endpoints. The client MUST validate the authorization server's TLS certificate as defined by [RFC6125] and in accordance with its requirements for server identity authentication.9.11.7.11. Credentials-Guessing Attacks The authorization server MUST prevent attackers from guessing access tokens, authorization codes, refresh tokens, resource owner passwords, and client credentials. The probability of an attacker guessing generated tokens (and other credentials not intended for handling by end-users) MUST be less than or equal to 2^(-128) and SHOULD be less than or equal to 2^(-160). The authorization server MUST utilize other means to protect credentials intended for end-user usage.9.12.7.12. Phishing Attacks Wide deployment of this and similar protocols may cause end-users to become inured to the practice of being redirected to websites where they are asked to enter their passwords. If end-users are not careful to verify the authenticity of these websites before entering their credentials, it will be possible for attackers to exploit this practice to steal resource owners' passwords. Service providers should attempt to educate end-users about the risks phishing attacks pose and should provide mechanisms that make it easy for end-users to confirm the authenticity of their sites. Client developers should consider the security implications of how they interact with theuser-agentuser agent (e.g., external, embedded), and the ability of the end-user to verify the authenticity of the authorization server. To reduce the risk of phishing attacks, the authorization servers MUST require the use of TLS on every endpoint used for end-user interaction.9.13.7.13. Fake External User-Agents in Native Apps The native app that is initiating the authorization request has a large degree of control over the user interface and can potentially present a fake externaluser-agent,user agent, that is, an embeddeduser-agentuser agent made to appear as an externaluser-agent.user agent. When all good actors are using externaluser-agents,user agents, the advantage is that it is possible for security experts to detect bad actors, as anyone faking an externaluser-agentuser agent is provably bad. On the other hand, if good and bad actors alike are using embeddeduser-agents,user agents, bad actors don't need to fake anything, making them harder to detect. Once a malicious app is detected, it may be possible to use this knowledge to blacklist the app's signature in malware scanning software, take removal action (in the case of apps distributed by app stores) and other steps to reduce the impact and spread of the malicious app. Authorization servers can also directly protect against fake externaluser-agentsuser agents by requiring an authentication factor only available to true externaluser-agents.user agents. Users who are particularly concerned about their security when using in-app browser tabs may also take the additional step of opening the request in the full browser from the in-app browser tab and complete the authorization there, as most implementations of the in-app browser tab pattern offer such functionality.9.14.7.14. Malicious External User-Agents in Native Apps If a malicious app is able to configure itself as the default handler for "https" scheme URIs in the operating system, it will be able to intercept authorization requests that use the default browser and abuse this position of trust for malicious ends such as phishing the user. This attack is not confined to OAuth; a malicious app configured in this way would present a general and ongoing risk to the user beyond OAuth usage by native apps. Many operating systems mitigate this issue by requiring an explicit user action to change the default handler for "http" and "https" scheme URIs.9.15.7.15. Cross-Site Request Forgery An attacker might attempt to inject a request to the redirect URI of the legitimate client on the victim's device, e.g., to cause the client to access resources under the attacker's control. This is a variant of an attack known as Cross-Site Request Forgery (CSRF). The traditional countermeasure are CSRF tokens that are bound to the user agent and passed in the "state" parameter to the authorization server as described in [RFC6819]. The same protection is provided by the "code_verifier" parameter or the OpenID Connect "nonce" value. When using "code_verifier" instead of "state" or "nonce" for CSRF protection, it is important to note that: * Clients MUST ensure that the AS supports the "code_challenge_method" intended to be used by the client. If an authorization server does not support the requested method, "state" or "nonce" MUST be used for CSRF protection instead. * If "state" is used for carrying application state, and integrity of its contents is a concern, clients MUST protect "state" against tampering and swapping. This can be achieved by binding the contents of state to the browser session and/or signed/encrypted state values [I-D.bradley-oauth-jwt-encoded-state]. AS therefore MUST provide a way to detect their supported code challenge methods either via AS metadata according to [RFC8414] or provide a deployment-specific way to ensure or determine support.9.16.7.16. Clickjacking As described in Section 4.4.1.9 of [RFC6819], the authorization request is susceptible to clickjacking. An attacker can use this vector to obtain the user's authentication credentials, change the scope of access granted to the client, and potentially access the user's resources. Authorization servers MUST prevent clickjacking attacks. Multiple countermeasures are described in [RFC6819], including the use of the X-Frame-Options HTTP response header field and frame-busting JavaScript. In addition to those, authorization servers SHOULD also use Content Security Policy (CSP) level 2 [CSP-2] or greater. To be effective, CSP must be used on the authorization endpoint and, if applicable, other endpoints used to authenticate the user and authorize the client (e.g., the device authorization endpoint, login pages, error pages, etc.). This prevents framing by unauthorized origins in user agents that support CSP. The client MAY permit being framed by some other origin than the one used in its redirection endpoint. For this reason, authorization servers SHOULD allow administrators to configure allowed origins for particular clients and/or for clients to register these dynamically. Using CSP allows authorization servers to specify multiple origins in a single response header field and to constrain these using flexible patterns (see [CSP-2] for details). Level 2 of this standard provides a robust mechanism for protecting against clickjacking by using policies that restrict the origin of frames (using "frame- ancestors") together with those that restrict the sources of scripts allowed to execute on an HTML page (by using "script-src"). A non- normative example of such a policy is shown in the following listing: "HTTP/1.1 200 OK Content-Security-Policy: frame-ancestors https://ext.example.org:8000 Content-Security-Policy: script-src 'self' X-Frame-Options: ALLOW-FROM https://ext.example.org:8000 ..." Because some user agents do not support [CSP-2], this technique SHOULD be combined with others, including those described in [RFC6819], unless such legacy user agents are explicitly unsupported by the authorization server. Even in such cases, additional countermeasures SHOULD still be employed.9.17.7.17. Code Injection and Input Validation A code injection attack occurs when an input or otherwise external variable is used by an application unsanitized and causes modification to the application logic. This may allow an attacker to gain access to the application device or its data, cause denial of service, or introduce a wide range of malicious side-effects. The authorization server and client MUST sanitize (and validate when possible) any value received - in particular, the value of the "state" and "redirect_uri" parameters.9.18.7.18. Open Redirectors The following attacks can occur when an AS or client has an open redirector. An open redirector is an endpoint that forwards a user's browser to an arbitrary URI obtained from a query parameter.9.18.1.7.18.1. Client as Open Redirector Clients MUST NOT expose open redirectors. Attackers may use open redirectors to produce URLs pointing to the client and utilize them to exfiltrate authorization codes and access tokens, as described in (#redir_uri_open_redir). Another abuse case is to produce URLs that appear to point to the client. This might trick users into trusting the URL and follow it in their browser. This can be abused for phishing. In order to prevent open redirection, clients should only redirect if the target URLs are whitelisted or if the origin and integrity of a request can be authenticated. Countermeasures against open redirection are described by OWASP [owasp_redir].9.18.2.7.18.2. Authorization Server as Open Redirector Just as with clients, attackers could try to utilize a user's trust in the authorization server (and its URL in particular) for performing phishing attacks. OAuth authorization servers regularly redirect users to other web sites (the clients), but must do so in a safe way. Section 4.1.2.1 already prevents open redirects by stating that the AS MUST NOT automatically redirect the user agent in case of an invalid combination of "client_id" and "redirect_uri". However, an attacker could also utilize a correctly registered redirect URI to perform phishing attacks. The attacker could, for example, register a client via dynamic client registration [RFC7591] and intentionally send an erroneous authorization request, e.g., by using an invalid scope value, thus instructing the AS to redirect the user agent to its phishing site. The AS MUST take precautions to prevent this threat. Based on its risk assessment, the AS needs to decide whether it can trust the redirect URI and SHOULD only automatically redirect the user agent if it trusts the redirect URI. If the URI is not trusted, the AS MAY inform the user and rely on the user to make the correct decision.9.19.7.19. Authorization Server Mix-Up Mitigation in Native Apps (TODO: merge this with the regular mix-up section when it is brought in) To protect against a compromised or malicious authorization server attacking another authorization server used by the same app, it is REQUIRED that a unique redirect URI is used for each authorization server used by the app (for example, by varying the path component), and that authorization responses are rejected if the redirect URI they were received on doesn't match the redirect URI in an outgoing authorization request. The native app MUST store the redirect URI used in the authorization request with the authorization session data (i.e., along with "state" and other related data) and MUST verify that the URI on which the authorization response was received exactly matches it. The requirement of Section9.2,7.3, specifically that authorization servers reject requests with URIs that don't match what was registered, is also required to prevent such attacks.9.20.7.20. Embedded User Agents in Native Apps Embeddeduser-agentsuser agents are a technically possible method for authorizing native apps. These embeddeduser-agentsuser agents are unsafe for use by third parties to the authorization server by definition, as the app that hosts the embeddeduser-agentuser agent can access the user's full authentication credential, not just the OAuth authorization grant that was intended for the app. In typical web-view-based implementations of embeddeduser-agents,user agents, the host application can record every keystroke entered in the login form to capture usernames and passwords, automatically submit forms to bypass user consent, and copy session cookies and use them to perform authenticated actions as the user. Even when used by trusted apps belonging to the same party as the authorization server, embeddeduser-agentsuser agents violate the principle of least privilege by having access to more powerful credentials than they need, potentially increasing the attack surface. Encouraging users to enter credentials in an embeddeduser-agentuser agent without the usual address bar and visible certificate validation features that browsers have makes it impossible for the user to know if they are signing in to the legitimate site; even when they are, it trains them that it's OK to enter credentials without validating the site first. Aside from the security concerns, embeddeduser-agentsuser agents do not share the authentication state with other apps or the browser, requiring the user to log in for every authorization request, which is often considered an inferior user experience.9.21.7.21. Other Recommendations Authorization servers SHOULD NOT allow clients to influence their "client_id" or "sub" value or any other claim if that can cause confusion with a genuine resource owner (see (#client_impersonating)).10.8. Native Applications Native applications are clients installed and executed on the device used by the resource owner (i.e., desktop application, native mobile application). Native applications require special consideration related to security, platform capabilities, and overall end-user experience. The authorization endpoint requires interaction between the client and the resource owner'suser-agent.user agent. The best current practice is to perform the OAuth authorization request in an externaluser-agentuser agent (typically the browser) rather than an embeddeduser-agentuser agent (such as one implemented with web-views). The native application can capture the response from the authorization server using a redirect URI with a scheme registered with the operating system to invoke the client as the handler, manual copy-and-paste of the credentials, running a local web server, installing auser-agentuser agent extension, or by providing a redirect URI identifying a server-hosted resource under the client's control, which in turn makes the response available to the native application. Previously, it was common for native apps to use embeddeduser-agentsuser agents (commonly implemented with web-views) for OAuth authorization requests. That approach has many drawbacks, including the host app being able to copy user credentials and cookies as well as the user needing to authenticate from scratch in each app. See Section9.207.20 for a deeper analysis of the drawbacks of using embeddeduser-agentsuser agents for OAuth. Native app authorization requests that use the browser are more secure and can take advantage of the user's authentication state. Being able to use the existing authentication session in the browser enables single sign-on, as users don't need to authenticate to the authorization server each time they use a new app (unless required by the authorization server policy). Supporting authorization flows between a native app and the browser is possible without changing the OAuth protocol itself, as the OAuth authorization request and response are already defined in terms of URIs. This encompasses URIs that can be used for inter-app communication. Some OAuth server implementations that assume all clients are confidential web clients will need to add an understanding of public native app clients and the types of redirect URIs they use to support this best practice.10.1.8.1. Using Inter-App URI Communication for OAuth in Native Apps Just as URIs are used for OAuth on the web to initiate the authorization request and return the authorization response to the requesting website, URIs can be used by native apps to initiate the authorization request in the device's browser and return the response to the requesting native app. By adopting the same methods used on the web for OAuth, benefits seen in the web context like the usability of a single sign-on session and the security of a separate authentication context are likewise gained in the native app context. Reusing the same approach also reduces the implementation complexity and increases interoperability by relying on standards-based web flows that are not specific to a particular platform. Native apps MUST use an externaluser-agentuser agent to perform OAuth authorization requests. This is achieved by opening the authorization request in the browser (detailed in Section10.2)8.2) and using a redirect URI that will return the authorization response back to the native app (defined in Section10.3). 10.2.8.3). 8.2. Initiating the Authorization Request from a Native App Native apps needing user authorization create an authorization request URI with the authorization code grant type per Section 4.1 using a redirect URI capable of being received by the native app. The function of the redirect URI for a native app authorization request is similar to that of a web-based authorization request. Rather than returning the authorization response to the OAuth client's server, the redirect URI used by a native app returns the response to the app. Several options for a redirect URI that will return the authorization response to the native app in different platforms are documented in Section10.3.8.3. Any redirect URI that allows the app to receive the URI and inspect its parameters is viable. After constructing the authorization request URI, the app uses platform-specific APIs to open the URI in an externaluser-agent.user agent. Typically, the externaluser-agentuser agent used is the default browser, that is, the application configured for handling "http" and "https" scheme URIs on the system; however, different browser selection criteria and other categories of externaluser-agentsuser agents MAY be used. This best practice focuses on the browser as the RECOMMENDED externaluser-agentuser agent for native apps. An externaluser-agentuser agent designed specifically for user authorization and capable of processing authorization requests and responses like a browser MAY also be used. Other externaluser-agents,user agents, such as a native app provided by the authorization server may meet the criteria set out in this best practice, including using the same redirect URI properties, but their use is out of scope for this specification. Some platforms support a browser feature known as "in-app browser tabs", where an app can present a tab of the browser within the app context without switching apps, but still retain key benefits of the browser such as a shared authentication state and security context. On platforms where they are supported, it is RECOMMENDED, for usability reasons, that apps use in-app browser tabs for the authorization request.10.3.8.3. Receiving the Authorization Response in a Native App There are several redirect URI options available to native apps for receiving the authorization response from the browser, the availability and user experience of which varies by platform. To fully support native apps, authorization servers MUST offer at least the three redirect URI options described in the following subsections to native apps. Native apps MAY use whichever redirect option suits their needs best, taking into account platform-specific implementation details.10.3.1.8.3.1. Private-Use URI Scheme Redirection Many mobile and desktop computing platforms support inter-app communication via URIs by allowing apps to register private-use URI schemes (sometimes colloquially referred to as "custom URL schemes") like "com.example.app". When the browser or another app attempts to load a URI with a private-use URI scheme, the app that registered it is launched to handle the request. To perform an authorization request with a private-use URI scheme redirect, the native app launches the browser with a standard authorization request, but one where the redirect URI utilizes a private-use URI scheme it registered with the operating system. When choosing a URI scheme to associate with the app, apps MUST use a URI scheme based on a domain name under their control, expressed in reverse order, as recommended by Section 3.8 of [RFC7595] for private-use URI schemes. For example, an app that controls the domain name "app.example.com" can use "com.example.app" as their scheme. Some authorization servers assign client identifiers based on domain names, for example, "client1234.usercontent.example.net", which can also be used as the domain name for the scheme when reversed in the same manner. A scheme such as "myapp", however, would not meet this requirement, as it is not based on a domain name. When there are multiple apps by the same publisher, care must be taken so that each scheme is unique within that group. On platforms that use app identifiers based on reverse-order domain names, those identifiers can be reused as the private-use URI scheme for the OAuth redirect to help avoid this problem. Following the requirements of Section 3.2 of [RFC3986], as there is no naming authority for private-use URI scheme redirects, only a single slash ("/") appears after the scheme component. A complete example of a redirect URI utilizing a private-use URI scheme is: com.example.app:/oauth2redirect/example-provider When the authorization server completes the request, it redirects to the client's redirect URI as it would normally. As the redirect URI uses a private-use URI scheme, it results in the operating system launching the native app, passing in the URI as a launch parameter. Then, the native app uses normal processing for the authorization response.10.3.2.8.3.2. Claimed "https" Scheme URI Redirection Some operating systems allow apps to claim "https" scheme [RFC7230] URIs in the domains they control. When the browser encounters a claimed URI, instead of the page being loaded in the browser, the native app is launched with the URI supplied as a launch parameter. Such URIs can be used as redirect URIs by native apps. They are indistinguishable to the authorization server from a regular web- based client redirect URI. An example is: https://app.example.com/oauth2redirect/example-provider As the redirect URI alone is not enough to distinguish public native app clients from confidential web clients, it is REQUIRED in Section9.27.3 that the client type be recorded during client registration to enable the server to determine the client type and act accordingly. App-claimed "https" scheme redirect URIs have some advantages compared to other native app redirect options in that the identity of the destination app is guaranteed to the authorization server by the operating system. For this reason, native apps SHOULD use them over the other options where possible.10.3.3.8.3.3. Loopback Interface Redirection Native apps that are able to open a port on the loopback network interface without needing special permissions (typically, those on desktop operating systems) can use the loopback interface to receive the OAuth redirect. Loopback redirect URIs use the "http" scheme and are constructed with the loopback IP literal and whatever port the client is listening on. That is, "http://127.0.0.1:{port}/{path}" for IPv4, and "http://[::1]:{port}/{path}" for IPv6. An example redirect using the IPv4 loopback interface with a randomly assigned port: http://127.0.0.1:51004/oauth2redirect/example-provider An example redirect using the IPv6 loopback interface with a randomly assigned port: http://[::1]:61023/oauth2redirect/example-provider The authorization server MUST allow any port to be specified at the time of the request for loopback IP redirect URIs, to accommodate clients that obtain an available ephemeral port from the operating system at the time of the request. Clients SHOULD NOT assume that the device supports a particular version of the Internet Protocol. It is RECOMMENDED that clients attempt to bind to the loopback interface using both IPv4 and IPv6 and use whichever is available.11.9. Browser-Based Apps Browser-based apps are are clients that run in a web browser, typically written in JavaScript, also known as "single-page apps". These types of apps have particular security considerations similar to native apps. TODO: Bring in the normative text of the browser-based apps BCP when it is finalized.12.10. Differences from OAuth 2.0 This draft consolidates the functionality in OAuth 2.0 [RFC6749], OAuth 2.0 for Native Apps ([RFC8252]), Proof Key for Code Exchange ([RFC7636]), OAuth 2.0 for Browser-Based Apps ([I-D.ietf-oauth-browser-based-apps]), OAuth Security Best Current Practice ([I-D.ietf-oauth-security-topics]), and Bearer Token Usage ([RFC6750]). Where a later draft updates or obsoletes functionality found in the original [RFC6749], that functionality in this draft is updated with the normative changes described in a later draft, or removed entirely. A non-normative list of changes from OAuth 2.0 is listed below: * The authorization code grant is extended with the functionality from PKCE ([RFC7636]) such that the default method of using the authorization code grant according to this specification requires the addition of the PKCE parameters * Redirect URIs must be compared using exact string matching as per Section 4.1.3 of [I-D.ietf-oauth-security-topics] * The Implicit grant ("response_type=token") is omitted from this specification as per Section 2.1.2 of [I-D.ietf-oauth-security-topics] * The Resource Owner Password Credentials grant is omitted from this specification as per Section 2.4 of [I-D.ietf-oauth-security-topics] * Bearer token usage omits the use of bearer tokens in the query string of URIs as per Section 4.3.2 of [I-D.ietf-oauth-security-topics] * Refresh tokens should either be sender-constrained or one-time use as per Section 4.12.2 of [I-D.ietf-oauth-security-topics]13.11. IANA Considerations This document does not require any IANA actions. All referenced registries are defined byRFC6749[RFC6749] and related documents that this work is based upon. No changes to those registries are required by this specification.14.12. References14.1.12.1. Normative References [BCP195]Sheffer, Y., Holz, R., and P.Saint-Andre, P., "Recommendations for Secure Use of Transport Layer Security (TLS)",n.d..2015. [I-D.ietf-oauth-security-topics] Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett, "OAuth 2.0 Security Best Current Practice", Work in Progress, Internet-Draft, draft-ietf-oauth-security-topics-16, 5 October 2020, <http://www.ietf.org/internet- drafts/draft-ietf-oauth-security-topics-16.txt>.topics-18, 13 April 2021, <https://www.ietf.org/archive/id/draft-ietf-oauth- security-topics-18.txt>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, DOI 10.17487/RFC2617, June 1999, <https://www.rfc-editor.org/info/rfc2617>. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, <https://www.rfc-editor.org/info/rfc2818>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>. [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>. [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <https://www.rfc-editor.org/info/rfc6125>. [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, <https://www.rfc-editor.org/info/rfc6749>. [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012, <https://www.rfc-editor.org/info/rfc6750>. [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, <https://www.rfc-editor.org/info/rfc7159>. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>. [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, <https://www.rfc-editor.org/info/rfc7231>. [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www.rfc-editor.org/info/rfc7234>. [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014, <https://www.rfc-editor.org/info/rfc7235>. [RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines and Registration Procedures for URI Schemes", BCP 35, RFC 7595, DOI 10.17487/RFC7595, June 2015, <https://www.rfc-editor.org/info/rfc7595>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, <https://www.rfc-editor.org/info/rfc8252>. [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/info/rfc8259>. [USASCII] Institute, A.N.S., "Coded Character Set -- 7-bit American Standard Code for Information Interchange, ANSI X3.4", 1986. [W3C.REC-html401-19991224] Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 Specification", World Wide Web Consortium Recommendation REC-html401-19991224, 24 December 1999, <https://www.w3.org/TR/1999/REC-html401-19991224>. [W3C.REC-xml-20081126] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", World Wide Web Consortium Recommendation REC- xml-20081126, 26 November 2008, <https://www.w3.org/TR/2008/REC-xml-20081126>.14.2.12.2. Informative References [CSP-2] "Content Security Policy Level 2", December 2016, <https://www.w3.org/TR/CSP2>. [I-D.bradley-oauth-jwt-encoded-state] Bradley, J., Lodderstedt, D. T., and H. Zandbelt, "Encoding claims in the OAuth 2 state parameter using a JWT", Work in Progress, Internet-Draft,draft-bradley-oauth-jwt- encoded-state-09,draft-bradley- oauth-jwt-encoded-state-09, 4 November 2018,<http://www.ietf.org/ internet-drafts/draft-bradley-oauth-jwt-encoded-state- 09.txt>.<https://www.ietf.org/archive/id/draft-bradley-oauth-jwt- encoded-state-09.txt>. [I-D.ietf-oauth-access-token-jwt] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens", Work in Progress, Internet-Draft, draft-ietf-oauth-access-token-jwt-11, 22 Januaryietf-oauth-access-token-jwt-13, 25 May 2021,<http://www.ietf.org/internet-drafts/draft-ietf-oauth- access-token-jwt-11.txt>.<https://www.ietf.org/archive/id/draft-ietf-oauth-access- token-jwt-13.txt>. [I-D.ietf-oauth-browser-based-apps] Parecki, A. and D. Waite, "OAuth 2.0 for Browser-Based Apps", Work in Progress, Internet-Draft, draft-ietf-oauth-browser-based-apps-07, 2 October 2020, <http://www.ietf.org/internet-drafts/draft-ietf-oauth- browser-based-apps-07.txt>.browser-based-apps-08, 17 May 2021, <https://www.ietf.org/archive/id/draft-ietf-oauth-browser- based-apps-08.txt>. [I-D.ietf-oauth-dpop] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T., Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof- of-Possession at the Application Layer (DPoP)", Work in Progress, Internet-Draft,draft-ietf-oauth-dpop-02, 18 November 2020, <http://www.ietf.org/internet-drafts/draft- ietf-oauth-dpop-02.txt>.draft-ietf-oauth-dpop-03, 7 April 2021, <https://www.ietf.org/archive/id/draft-ietf- oauth-dpop-03.txt>. [I-D.ietf-oauth-par] Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D., and F. Skokan, "OAuth 2.0 Pushed Authorization Requests", Work in Progress, Internet-Draft,draft-ietf-oauth-par-05, 14 December 2020, <http://www.ietf.org/internet-drafts/ draft-ietf-oauth-par-05.txt>.draft-ietf-oauth-par-10, 29 July 2021, <https://www.ietf.org/archive/id/draft-ietf- oauth-par-10.txt>. [I-D.ietf-oauth-rar] Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0 Rich Authorization Requests", Work in Progress, Internet- Draft,draft-ietf-oauth-rar-03, 18 October 2020, <http://www.ietf.org/internet-drafts/draft-ietf-oauth-rar- 03.txt>.draft-ietf-oauth-rar-05, 15 May 2021, <https://www.ietf.org/archive/id/draft-ietf-oauth-rar- 05.txt>. [I-D.ietf-oauth-token-binding] Jones,M.,M. B., Campbell, B., Bradley, J., and W. Denniss, "OAuth 2.0 Token Binding", Work in Progress, Internet- Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,<http://www.ietf.org/internet-drafts/draft-ietf-oauth- token-binding-08.txt>.<https://www.ietf.org/archive/id/draft-ietf-oauth-token- binding-08.txt>. [NIST800-63] Burr, W., Dodson, D., Newton, E., Perlner, R., Polk, T., Gupta, S., and E. Nabbus, "NIST Special Publication 800-63-1, INFORMATION SECURITY", December 2011, <http://csrc.nist.gov/publications/>. [OMAP] Huff, J., Schlacht, D., Nadalin, A., Simmons, J., Rosenberg, P., Madsen, P., Ace, T., Rickelton-Abdi, C., and B. Boyer, "Online Multimedia Authorization Protocol: An Industry Standard for Authorized Access to Internet Multimedia Resources", April 2012, <https://www.oatc.us/Standards/Download-Standards>. [OpenID] Sakimora, N., Bradley, J., Jones, M., de Medeiros, B., and C. Mortimore, "OpenID Connect Core 1.0", November 2014, <https://openiD.net/specs/openiD-connect-core-1_0.html>. [OpenID.Messages] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., Mortimore, C., and E. Jay, "OpenID Connect Messages 1.0", June 2012, <http://openid.net/specs/openid-connect- messages-1_0.html>. [owasp_redir] "OWASP Cheat Sheet Series - Unvalidated Redirects and Forwards", 2020, <https://cheatsheetseries.owasp.org/cheatsheets/ Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011, <https://www.rfc-editor.org/info/rfc6265>. [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0 Threat Model and Security Considerations", RFC 6819, DOI 10.17487/RFC6819, January 2013, <https://www.rfc-editor.org/info/rfc6819>. [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, August 2013, <https://www.rfc-editor.org/info/rfc7009>.[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>. [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014, <https://www.rfc-editor.org/info/rfc7235>.[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, <https://www.rfc-editor.org/info/rfc7519>. [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", RFC 7591, DOI 10.17487/RFC7591, July 2015, <https://www.rfc-editor.org/info/rfc7591>. [RFC7592] Richer, J., Ed., Jones, M., Bradley, J., and M. Machulak, "OAuth 2.0 Dynamic Client Registration Management Protocol", RFC 7592, DOI 10.17487/RFC7592, July 2015, <https://www.rfc-editor.org/info/rfc7592>. [RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key for Code Exchange by OAuth Public Clients", RFC 7636, DOI 10.17487/RFC7636, September 2015, <https://www.rfc-editor.org/info/rfc7636>. [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", RFC 7662, DOI 10.17487/RFC7662, October 2015, <https://www.rfc-editor.org/info/rfc7662>. [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, June 2018, <https://www.rfc-editor.org/info/rfc8414>. [RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig, "OAuth 2.0 Device Authorization Grant", RFC 8628, DOI 10.17487/RFC8628, August 2019, <https://www.rfc-editor.org/info/rfc8628>. [RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T. Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens", RFC 8705, DOI 10.17487/RFC8705, February 2020, <https://www.rfc-editor.org/info/rfc8705>. [RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707, February 2020, <https://www.rfc-editor.org/info/rfc8707>. Appendix A. Augmented Backus-Naur Form (ABNF) Syntax This section provides Augmented Backus-Naur Form (ABNF) syntax descriptions for the elements defined in this specification using the notation of [RFC5234]. The ABNF below is defined in terms of Unicode code points [W3C.REC-xml-20081126]; these characters are typically encoded in UTF-8. Elements are presented in the order first defined. Some of the definitions that follow use the "URI-reference" definition from [RFC3986]. Some of the definitions that follow use these common definitions: VSCHAR = %x20-7E NQCHAR = %x21 / %x23-5B / %x5D-7E NQSCHAR = %x20-21 / %x23-5B / %x5D-7E UNICODECHARNOCRLF = %x09 /%x20-7E / %x80-D7FF / %xE000-FFFD / %x10000-10FFFF (The UNICODECHARNOCRLF definition is based upon the Char definition in Section 2.2 of [W3C.REC-xml-20081126], but omitting the Carriage Return and Linefeed characters.) A.1. "client_id" Syntax The "client_id" element is defined in Section2.3.1:2.4.1: client-id = *VSCHAR A.2. "client_secret" Syntax The "client_secret" element is defined in Section2.3.1:2.4.1: client-secret = *VSCHAR A.3. "response_type" Syntax The "response_type" element is defined in Section3.1.14.1.1 and Section8.4:6.4: response-type = response-name *( SP response-name ) response-name = 1*response-char response-char = "_" / DIGIT / ALPHA A.4. "scope" Syntax The "scope" element is defined in Section3.3:3.2.2.1: scope = scope-token *( SP scope-token ) scope-token = 1*NQCHAR A.5. "state" Syntax The "state" element is defined in Section 4.1.1, Section 4.1.2, and Section 4.1.2.1: state = 1*VSCHAR A.6. "redirect_uri" Syntax The "redirect_uri" element is defined in Section 4.1.1, and Section 4.1.3: redirect-uri = URI-reference A.7. "error" Syntax The "error" element is defined in Sections Section 4.1.2.1, Section5.2,3.2.3.1, 7.2, and 8.5: error = 1*NQSCHAR A.8. "error_description" Syntax The "error_description" element is defined in Sections Section 4.1.2.1, Section5.2,3.2.3.1, and Section7.3:5.3: error-description = 1*NQSCHAR A.9. "error_uri" Syntax The "error_uri" element is defined in Sections Section 4.1.2.1, Section5.2,3.2.3.1, and 7.2: error-uri = URI-reference A.10. "grant_type" Syntax The "grant_type" element is defined inSections Section 4.1.3, Section 4.2.3, Section 4.2.2,Section4.3, andSection6:3.2.2: grant-type = grant-name / URI-reference grant-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA A.11. "code" Syntax The "code" element is defined in Section 4.1.3: code = 1*VSCHAR A.12. "access_token" Syntax The "access_token" element is defined in Section4.2.3 and Section 5.1:3.2.3: access-token = 1*VSCHAR A.13. "token_type" Syntax The "token_type" element is defined in Section5.1,3.2.3, and Section8.1:6.1: token-type = type-name / URI-reference type-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA A.14. "expires_in" Syntax The "expires_in" element is defined in Section5.1:3.2.3: expires-in = 1*DIGIT A.15. "refresh_token" Syntax The "refresh_token" element is defined in Section5.13.2.3 and Section6:4.3: refresh-token = 1*VSCHAR A.16. Endpoint Parameter Syntax The syntax for new endpoint parameters is defined in Section8.2:6.2: param-name = 1*name-char name-char = "-" / "." / "_" / DIGIT / ALPHA A.17. "code_verifier" Syntax ABNF for "code_verifier" is as follows. code-verifier = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39 A.18. "code_challenge" Syntax ABNF for "code_challenge" is as follows. code-challenge = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39 Appendix B. Use of application/x-www-form-urlencoded Media Type At the time of publication of this specification, the "application/x- www-form-urlencoded" media type was defined in Section 17.13.4 of [W3C.REC-html401-19991224] but not registered in the IANA MIME Media Types registry (http://www.iana.org/assignments/media-types (http://www.iana.org/assignments/media-types)). Furthermore, that definition is incomplete, as it does not consider non-US-ASCII characters. To address this shortcoming when generating payloads using this media type, names and values MUST be encoded using the UTF-8 character encoding scheme [RFC3629] first; the resulting octet sequence then needs to be further encoded using the escaping rules defined in [W3C.REC-html401-19991224]. When parsing data from a payload using this media type, the names and values resulting from reversing the name/value encoding consequently need to be treated as octet sequences, to be decoded using the UTF-8 character encoding scheme. For example, the value consisting of the six Unicode code points (1) U+0020 (SPACE), (2) U+0025 (PERCENT SIGN), (3) U+0026 (AMPERSAND), (4) U+002B (PLUS SIGN), (5) U+00A3 (POUND SIGN), and (6) U+20AC (EURO SIGN) would be encoded into the octet sequence below (using hexadecimal notation): 20 25 26 2B C2 A3 E2 82 AC and then represented in the payload as: +%25%26%2B%C2%A3%E2%82%AC Appendix C. Extensions Below is a list of well-established extensions at the time of publication: * [RFC8628]: OAuth 2.0 Device Authorization Grant - The Device Authorization Grant (formerly known as the Device Flow) is an extension that enables devices with no browser or limited input capability to obtain an access token. This is commonly used by smart TV apps, or devices like hardware video encoders that can stream video to a streaming video service. * [RFC8414]: Authorization Server Metadata - Authorization Server Metadata (also known as OAuth Discovery) defines an endpoint clients can use to look up the information needed to interact with a particular OAuth server, such as the location of the authorization and token endpoints and the supported grant types. * [RFC8707]: Resource Indicators - Provides a way for the client to explicitly signal to the authorization server where it intends to use the access token it is requesting. * [RFC7591]: Dynamic Client Registration - Dynamic Client Registration provides a mechanism for programmatically registering clients with an authorization server. * [RFC7592]: Dynamic Client Management - Dynamic Client Management provides a mechanism for updating dynamically registered client information. * [I-D.ietf-oauth-access-token-jwt]: JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens - This specification defines a profile for issuing OAuth access tokens in JSON Web Token (JWT) format. * [RFC8705]: Mutual TLS - Mutual TLS describes a mechanism of binding access tokens and refresh tokens to the clients they were issued to, as well as a client authentication mechanism, via TLS certificate authentication. * [RFC7662]: Token Introspection - The Token Introspection extension defines a mechanism for resource servers to obtain information about access tokens. * [RFC7009]: Token Revocation - The Token Revocation extension defines a mechanism for clients to indicate to the authorization server that an access token is no longer needed. * [I-D.ietf-oauth-par]: Pushed Authorization Requests - The Pushed AuthorizationRequstsRequests extension describes a technique of initiating an OAuth flow from the back channel, providing better security and more flexibility for building complex authorization requests. * [I-D.ietf-oauth-rar]: Rich Authorization Requests - Rich Authorization Requests specifies a new parameter "authorization_details" that is used to carry fine-grained authorization data in the OAuth authorization request. Appendix D. Acknowledgements TBD Appendix E. Document History [[ To be removed from the final specification ]] -03 * refactored structure -02 -01 -00 * initial revision Authors' Addresses Dick Hardt SignIn.Org Email: dick.hardt@gmail.com Aaron Parecki Okta Email: aaron@parecki.com URI: https://aaronparecki.com Torsten Lodderstedt yes.com Email: torsten@lodderstedt.net