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Versions: (draft-lodderstedt-oauth-security-topics) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

Web Authorization Protocol                                T. Lodderstedt
Internet-Draft                                                   yes.com
Intended status: Best Current Practice                        J. Bradley
Expires: August 13, 2020                                          Yubico
                                                             A. Labunets

                                                                 D. Fett
                                                                 yes.com
                                                       February 10, 2020


                OAuth 2.0 Security Best Current Practice
                  draft-ietf-oauth-security-topics-14

Abstract

   This document describes best current security practice for OAuth 2.0.
   It updates and extends the OAuth 2.0 Security Threat Model to
   incorporate practical experiences gathered since OAuth 2.0 was
   published and covers new threats relevant due to the broader
   application of OAuth 2.0.

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 on August 13, 2020.

Copyright Notice

   Copyright (c) 2020 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions and Terminology . . . . . . . . . . . . . . .   4
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Protecting Redirect-Based Flows . . . . . . . . . . . . .   5
       2.1.1.  Authorization Code Grant  . . . . . . . . . . . . . .   6
       2.1.2.  Implicit Grant  . . . . . . . . . . . . . . . . . . .   6
     2.2.  Token Replay Prevention . . . . . . . . . . . . . . . . .   7
     2.3.  Access Token Privilege Restriction  . . . . . . . . . . .   7
     2.4.  Resource Owner Password Credentials Grant . . . . . . . .   8
     2.5.  Client Authentication . . . . . . . . . . . . . . . . . .   8
     2.6.  Other Recommendations . . . . . . . . . . . . . . . . . .   8
   3.  The Updated OAuth 2.0 Attacker Model  . . . . . . . . . . . .   8
   4.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .  10
     4.1.  Insufficient Redirect URI Validation  . . . . . . . . . .  11
       4.1.1.  Redirect URI Validation Attacks on Authorization Code
               Grant . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.2.  Redirect URI Validation Attacks on Implicit Grant . .  13
       4.1.3.  Countermeasures . . . . . . . . . . . . . . . . . . .  14
     4.2.  Credential Leakage via Referer Headers  . . . . . . . . .  15
       4.2.1.  Leakage from the OAuth Client . . . . . . . . . . . .  15
       4.2.2.  Leakage from the Authorization Server . . . . . . . .  15
       4.2.3.  Consequences  . . . . . . . . . . . . . . . . . . . .  16
       4.2.4.  Countermeasures . . . . . . . . . . . . . . . . . . .  16
     4.3.  Credential Leakage via Browser History  . . . . . . . . .  17
       4.3.1.  Authorization Code in Browser History . . . . . . . .  17
       4.3.2.  Access Token in Browser History . . . . . . . . . . .  17
     4.4.  Mix-Up Attacks  . . . . . . . . . . . . . . . . . . . . .  18
       4.4.1.  Attack Description  . . . . . . . . . . . . . . . . .  18
       4.4.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  20
     4.5.  Authorization Code Injection  . . . . . . . . . . . . . .  21
       4.5.1.  Attack Description  . . . . . . . . . . . . . . . . .  21
       4.5.2.  Discussion  . . . . . . . . . . . . . . . . . . . . .  22
       4.5.3.  Countermeasures . . . . . . . . . . . . . . . . . . .  23
       4.5.4.  Limitations . . . . . . . . . . . . . . . . . . . . .  24
     4.6.  Access Token Injection  . . . . . . . . . . . . . . . . .  24
       4.6.1.  Countermeasures . . . . . . . . . . . . . . . . . . .  25
     4.7.  Cross Site Request Forgery  . . . . . . . . . . . . . . .  25
       4.7.1.  Countermeasures . . . . . . . . . . . . . . . . . . .  25
     4.8.  Access Token Leakage at the Resource Server . . . . . . .  25



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       4.8.1.  Access Token Phishing by Counterfeit Resource Server   26
       4.8.2.  Compromised Resource Server . . . . . . . . . . . . .  31
     4.9.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  31
       4.9.1.  Client as Open Redirector . . . . . . . . . . . . . .  32
       4.9.2.  Authorization Server as Open Redirector . . . . . . .  32
     4.10. 307 Redirect  . . . . . . . . . . . . . . . . . . . . . .  32
     4.11. TLS Terminating Reverse Proxies . . . . . . . . . . . . .  33
     4.12. Refresh Token Protection  . . . . . . . . . . . . . . . .  34
       4.12.1.  Discussion . . . . . . . . . . . . . . . . . . . . .  34
       4.12.2.  Recommendations  . . . . . . . . . . . . . . . . . .  35
     4.13. Client Impersonating Resource Owner . . . . . . . . . . .  36
       4.13.1.  Countermeasures  . . . . . . . . . . . . . . . . . .  36
     4.14. Clickjacking  . . . . . . . . . . . . . . . . . . . . . .  36
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  37
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  38
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  38
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  39
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  46

1.  Introduction

   Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 ("OAuth"
   in the following) has gotten massive traction in the market and
   became the standard for API protection and the basis for federated
   login using OpenID Connect [OpenID].  While OAuth is used in a
   variety of scenarios and different kinds of deployments, the
   following challenges can be observed:

   o  OAuth implementations are being attacked through known
      implementation weaknesses and anti-patterns.  Although most of
      these threats are discussed in the OAuth 2.0 Threat Model and
      Security Considerations [RFC6819], continued exploitation
      demonstrates a need for more specific recommendations, easier to
      implement mitigations, and more defense in depth.

   o  OAuth is being used in environments with higher security
      requirements than considered initially, such as Open Banking,
      eHealth, eGovernment, and Electronic Signatures.  Those use cases
      call for stricter guidelines and additional protection.

   o  OAuth is being used in much more dynamic setups than originally
      anticipated, creating new challenges with respect to security.
      Those challenges go beyond the original scope of [RFC6749],
      [RFC6750], and [RFC6819].




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      OAuth initially assumed a static relationship between client,
      authorization server and resource servers.  The URLs of AS and RS
      were known to the client at deployment time and built an anchor
      for the trust relationship among those parties.  The validation
      whether the client talks to a legitimate server was based on TLS
      server authentication (see [RFC6819], Section 4.5.4).  With the
      increasing adoption of OAuth, this simple model dissolved and, in
      several scenarios, was replaced by a dynamic establishment of the
      relationship between clients on one side and the authorization and
      resource servers of a particular deployment on the other side.
      This way, the same client could be used to access services of
      different providers (in case of standard APIs, such as e-mail or
      OpenID Connect) or serve as a frontend to a particular tenant in a
      multi-tenancy environment.  Extensions of OAuth, such as the OAuth
      2.0 Dynamic Client Registration Protocol [RFC7591] and OAuth 2.0
      Authorization Server Metadata [RFC8414] were developed in order to
      support the usage of OAuth in dynamic scenarios.

   o  Technology has changed.  For example, the way browsers treat
      fragments when redirecting requests has changed, and with it, the
      implicit grant's underlying security model.

   This document provides updated security recommendations to address
   these challenges.  It does not supplant the security advice given in
   [RFC6749], [RFC6750], and [RFC6819], but complements those documents.

1.1.  Structure

   The remainder of this document is organized as follows: The next
   section summarizes the most important recommendations of the OAuth
   working group for every OAuth implementor.  Afterwards, the updated
   the OAuth attacker model is presented.  Subsequently, a detailed
   analysis of the threats and implementation issues that can be found
   in the wild today is given along with a discussion of potential
   countermeasures.

1.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to 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 terms "access token", "authorization
   endpoint", "authorization grant", "authorization server", "client",
   "client identifier" (client ID), "protected resource", "refresh




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   token", "resource owner", "resource server", and "token endpoint"
   defined by OAuth 2.0 [RFC6749].

2.  Recommendations

   This section describes the set of security mechanisms the OAuth
   working group recommends to OAuth implementers.

2.1.  Protecting Redirect-Based Flows

   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 Section 4.1).  It can also help to
   detect mix-up attacks (see Section 4.4).

   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 Section 4.9.1.

   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 PKCE [RFC7636]
   MAY rely the CSRF protection provided by PKCE.  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 Section 4.7.1).

   In order to prevent mix-up attacks (see Section 4.4), 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 authorization
   request was received from the correct authorization server.  Clients
   MUST ensure that the subsequent 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 Section 4.10 for details).




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2.1.1.  Authorization Code Grant

   Clients MUST prevent injection (replay) of authorization codes into
   the authorization response by attackers.  The use of PKCE [RFC7636]
   is RECOMMENDED to this end.  The OpenID Connect "nonce" parameter and
   ID Token Claim [OpenID] MAY be used as well.  The PKCE challenge or
   OpenID Connect "nonce" MUST be transaction-specific and securely
   bound to the client and the user agent in which the transaction was
   started.

   Note: although PKCE so far was designed as a mechanism to protect
   native apps, this advice applies to all kinds of OAuth clients,
   including web applications.

   When using PKCE, clients SHOULD use PKCE code challenge methods that
   do not expose the PKCE verifier in the authorization request.
   Otherwise, attackers that can read the authorization request (cf.
   Attacker A4 in Section 3) can break the security provided by PKCE.
   Currently, "S256" is the only such method.

   Authorization servers MUST support PKCE [RFC7636].

   Authorization servers MUST provide a way to detect their support for
   PKCE.  To this end, they MUST either (a) publish the element
   "code_challenge_methods_supported" in their AS metadata ([RFC8418])
   containing the supported PKCE challenge methods (which can be used by
   the client to detect PKCE support) or (b) provide a deployment-
   specific way to ensure or determine PKCE support by the AS.

2.1.2.  Implicit Grant

   The implicit grant (response type "token") and other response types
   causing the authorization server to issue access tokens in the
   authorization response are vulnerable to access token leakage and
   access token replay as described in Section 4.1, Section 4.2,
   Section 4.3, and Section 4.6.

   Moreover, no viable mechanism exists to cryptographically bind access
   tokens issued in the authorization response to a certain client as it
   is recommended in Section 2.2.  This makes replay detection for such
   access tokens at resource servers impossible.

   In order to avoid these issues, clients SHOULD NOT use the implicit
   grant (response type "token") or other response types issuing access
   tokens in the authorization response, unless access token injection
   in the authorization response is prevented and the aforementioned
   token leakage vectors are mitigated.




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   Clients SHOULD instead use the response type "code" (aka
   authorization code grant type) as specified in Section 2.1.1 or any
   other response type that causes the authorization server to issue
   access tokens in the token response, such as the "code id_token"
   response type.  This allows the authorization server to detect replay
   attempts by attackers and generally reduces the attack surface since
   access tokens are not exposed in URLs.  It also allows the
   authorization server to sender-constrain the issued tokens (see next
   section).

2.2.  Token Replay Prevention

   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 token at the recipient (e.g., a resource server).

   Authorization and resource servers SHOULD use mechanisms for sender-
   constrained access tokens to prevent token replay as described in
   Section 4.8.1.1.2.  The use of Mutual TLS for OAuth 2.0 [RFC8705] is
   RECOMMENDED.  Refresh tokens MUST be sender-constrained or use
   refresh token rotation as described in Section 4.12.

   It is RECOMMENDED to use end-to-end TLS.  If TLS traffic needs to be
   terminated at an intermediary, refer to Section 4.11 for further
   security advice.

2.3.  Access Token Privilege Restriction

   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
   [RFC6749] and [I-D.ietf-oauth-resource-indicators], respectively, to
   determine the resource server they want to access.




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   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" as specified in [RFC6749] and
   "authorization_details" as specified in [I-D.ietf-oauth-rar] to
   determine those resources and/or actions.

2.4.  Resource Owner Password Credentials Grant

   The resource owner password credentials grant MUST NOT be used.  This
   grant type insecurely exposes the credentials of the resource owner
   to the client.  Even if the client is benign, this results in an
   increased attack surface (credentials can leak in more places than
   just the AS) and users are trained to enter their credentials in
   places other than the AS.

   Furthermore, adapting the resource owner password credentials grant
   to two-factor authentication, authentication with cryptographic
   credentials (cf.  WebCrypto [webcrypto], WebAuthn [webauthn]), and
   authentication processes that require multiple steps can be hard or
   impossible.

2.5.  Client Authentication

   Authorization servers SHOULD use client authentication if possible.

   It is RECOMMENDED to use asymmetric (public-key based) methods for
   client authentication such as mTLS [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 methods more robust against a number of attacks.

2.6.  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 Section 4.13).

3.  The Updated OAuth 2.0 Attacker Model

   In [RFC6819], an attacker model is laid out that describes the
   capabilities of attackers against which OAuth deployments must be
   protected.  In the following, this attacker model is updated to



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   account for the potentially dynamic relationships involving multiple
   parties (as described in Section 1), to include new types of
   attackers and to define the attacker model more clearly.

   OAuth MUST ensure that the authorization of the resource owner (RO)
   (with a user agent) at the authorization server (AS) and the
   subsequent usage of the access token at the resource server (RS) is
   protected at least against the following attackers:

   o  (A1) Web Attackers that can set up and operate an arbitrary number
      of network endpoints including browsers and servers (except for
      the concrete RO, AS, and RS).  Web attackers may set up web sites
      that are visited by the RO, operate their own user agents, and
      participate in the protocol.
      Web attackers may, in particular, operate OAuth clients that are
      registered at AS, and operate their own authorization and resource
      servers that can be used (in parallel) by the RO and other
      resource owners.
      It must also be assumed that web attackers can lure the user to
      open arbitrary attacker-chosen URIs at any time.  In practice,
      this can be achieved in many ways, for example, by injecting
      malicious advertisements into advertisement networks, or by
      sending legit-looking emails.
      Web attackers can use their own user credentials to create new
      messages as well as any secrets they learned previously.  For
      example, if a web attacker learns an authorization code of a user
      through a misconfigured redirect URI, the web attacker can then
      try to redeem that code for an access token.
      They cannot, however, read or manipulate messages that are not
      targeted towards them (e.g., sent to a URL controlled by a non-
      attacker controlled AS).

   o  (A2) Network Attackers that additionally have full control over
      the network over which protocol participants communicate.  They
      can eavesdrop on, manipulate, and spoof messages, except when
      these are properly protected by cryptographic methods (e.g., TLS).
      Network attackers can also block arbitrary messages.

   While an example for a web attacker would be a customer of an
   internet service provider, network attackers could be the internet
   service provider itself, an attacker in a public (wifi) network using
   ARP spoofing, or a state-sponsored attacker with access to internet
   exchange points, for instance.

   These attackers conform to the attacker model that was used in formal
   analysis efforts for OAuth [arXiv.1601.01229].  This is a minimal
   attacker model.  Implementers MUST take into account all possible
   attackers in the environment in which their OAuth implementations are



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   expected to run.  Previous attacks on OAuth have shown that OAuth
   deployments SHOULD in particular consider the following, stronger
   attackers in addition to those listed above:

   o  (A3) Attackers that can read, but not modify, the contents of the
      authorization response (i.e., the authorization response can leak
      to an attacker).
      Examples for such attacks include open redirector attacks,
      problems existing on mobile operating systems (where different
      apps can register themselves on the same URI), mix-up attacks (see
      Section 4.4), where the client is tricked into sending credentials
      to a attacker-controlled AS, and the fact that URLs are often
      stored/logged by browsers (history), proxy servers, and operating
      systems.

   o  (A4) Attackers that can read, but not modify, the contents of the
      authorization request (i.e., the authorization request can leak,
      in the same manner as above, to an attacker).

   o  (A5) Attackers that can acquire an access token issued by AS.  For
      example, a resource server can be compromised by an attacker, an
      access token may be sent to an attacker-controlled resource server
      due to a misconfiguration, or an RO is social-engineered into
      using a attacker-controlled RS.  See also Section 4.8.2.

   (A3), (A4) and (A5) typically occur together with either (A1) or
   (A2).

   Note that in this attacker model, an attacker (see A1) can be a RO or
   act as one.  For example, an attacker can use his own browser to
   replay tokens or authorization codes obtained by any of the attacks
   described above at the client or RS.

   This document focusses on threats resulting from these attackers.
   Attacks in an even stronger attacker model are discussed, for
   example, in [arXiv.1901.11520].

4.  Attacks and Mitigations

   This section gives a detailed description of attacks on OAuth
   implementations, along with potential countermeasures.  Attacks and
   mitigations already covered in [RFC6819] are not listed here, except
   where new recommendations are made.








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4.1.  Insufficient Redirect URI Validation

   Some authorization servers allow clients to register redirect URI
   patterns instead of complete redirect URIs.  The authorization
   servers then match the redirect URI parameter value at the
   authorization endpoint against the registered patterns at runtime.
   This approach allows clients to encode transaction state into
   additional redirect URI parameters or to register a single pattern
   for multiple redirect URIs.

   This approach turned out to be more complex to implement and more
   error prone to manage than exact redirect URI matching.  Several
   successful attacks exploiting flaws in the pattern matching
   implementation or concrete configurations have been observed in the
   wild . Insufficient validation of the redirect URI effectively breaks
   client identification or authentication (depending on grant and
   client type) and allows the attacker to obtain an authorization code
   or access token, either

   o  by directly sending the user agent to a URI under the attackers
      control, or

   o  by exposing the OAuth credentials to an attacker by utilizing an
      open redirector at the client in conjunction with the way user
      agents handle URL fragments.

   These attacks are shown in detail in the following subsections.

4.1.1.  Redirect URI Validation Attacks on Authorization Code Grant

   For a client using the grant type code, an attack may work as
   follows:

   Assume the redirect URL pattern "https://*.somesite.example/*" is
   registered for the client with the client ID "s6BhdRkqt3".  The
   intention is to allow any subdomain of "somesite.example" to be a
   valid redirect URI for the client, for example
   "https://app1.somesite.example/redirect".  A naive implementation on
   the authorization server, however, might interpret the wildcard "*"
   as "any character" and not "any character valid for a domain name".
   The authorization server, therefore, might permit
   "https://attacker.example/.somesite.example" as a redirect URI,
   although "attacker.example" is a different domain potentially
   controlled by a malicious party.

   The attack can then be conducted as follows:





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   First, the attacker needs to trick the user into opening a tampered
   URL in his browser that launches a page under the attacker's control,
   say "https://www.evil.example" (see Attacker A1.)

   This URL initiates the following authorization request with the
   client ID of a legitimate client to the authorization endpoint (line
   breaks for display only):

   GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
        &redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example
        HTTP/1.1
   Host: server.somesite.example

   The authorization server validates the redirect URI and compares it
   to the registered redirect URL patterns for the client "s6BhdRkqt3".
   The authorization request is processed and presented to the user.

   If the user does not see the redirect URI or does not recognize the
   attack, the code is issued and immediately sent to the attacker's
   domain.  If an automatic approval of the authorization is enabled
   (which is not recommended for public clients according to [RFC6749]),
   the attack can be performed even without user interaction.

   If the attacker impersonated a public client, the attacker can
   exchange the code for tokens at the respective token endpoint.

   This attack will not work as easily for confidential clients, since
   the code exchange requires authentication with the legitimate
   client's secret.  The attacker can, however, use the legitimate
   confidential client to redeem the code by performing an authorization
   code injection attack, see Section 4.5.

   Note: Vulnerabilities of this kind can also exist if the
   authorization server handles wildcards properly.  For example, assume
   that the client registers the redirect URL pattern
   "https://*.somesite.example/*" and the authorization server
   interprets this as "allow redirect URIs pointing to any host residing
   in the domain "somesite.example"".  If an attacker manages to
   establish a host or subdomain in "somesite.example", he can
   impersonate the legitimate client.  This could be caused, for
   example, by a subdomain takeover attack [subdomaintakeover], where an
   outdated CNAME record (say, "external-service.somesite.example")
   points to an external DNS name that does no longer exist (say,
   "customer-abc.service.example") and can be taken over by an attacker
   (e.g., by registering as "customer-abc" with the external service).






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4.1.2.  Redirect URI Validation Attacks on Implicit Grant

   The attack described above works for the implicit grant as well.  If
   the attacker is able to send the authorization response to a URI
   under his control, he will directly get access to the fragment
   carrying the access token.

   Additionally, implicit clients can be subject to a further kind of
   attack.  It utilizes the fact that user agents re-attach fragments to
   the destination URL of a redirect if the location header does not
   contain a fragment (see [RFC7231], Section 9.5).  The attack
   described here combines this behavior with the client as an open
   redirector (see Section 4.9.1) in order to get access to access
   tokens.  This allows circumvention even of very narrow redirect URI
   patterns, but not strict URL matching.

   Assume the registered URL pattern for client "s6BhdRkqt3" is
   "https://client.somesite.example/cb?*", i.e., any parameter is
   allowed for redirects to "https://client.somesite.example/cb".
   Unfortunately, the client exposes an open redirector.  This endpoint
   supports a parameter "redirect_to" which takes a target URL and will
   send the browser to this URL using an HTTP Location header redirect
   303.

   The attack can now be conducted as follows:

   First, and as above, the attacker needs to trick the user into
   opening a tampered URL in his browser that launches a page under the
   attacker's control, say "https://www.evil.example".

   Afterwards, the website initiates an authorization request that is
   very similar to the one in the attack on the code flow.  Different to
   above, it utilizes the open redirector by encoding
   "redirect_to=https://attacker.example" into the parameters of the
   redirect URI and it uses the response type "token" (line breaks for
   display only):

   GET /authorize?response_type=token&state=9ad67f13
       &client_id=s6BhdRkqt3
       &redirect_uri=https%3A%2F%2Fclient.somesite.example
        %2Fcb%26redirect_to%253Dhttps%253A%252F
        %252Fattacker.example%252F HTTP/1.1
   Host: server.somesite.example

   Now, since the redirect URI matches the registered pattern, the
   authorization server permits the request and sends the resulting
   access token in a 303 redirect (some response parameters omitted for
   readability):



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   HTTP/1.1 303 See Other
   Location: https://client.somesite.example/cb?
             redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb
             #access_token=2YotnFZFEjr1zCsicMWpAA&...

   At example.com, the request arrives at the open redirector.  The
   endpoint will read the redirect parameter and will issue an HTTP 303
   Location header redirect to the URL "https://attacker.example/".

   HTTP/1.1 303 See Other
   Location: https://attacker.example/

   Since the redirector at client.somesite.example does not include a
   fragment in the Location header, the user agent will re-attach the
   original fragment "#access_token=2YotnFZFEjr1zCsicMWpAA&..." to
   the URL and will navigate to the following URL:

   https://attacker.example/#access_token=2YotnFZFEjr1z...

   The attacker's page at "attacker.example" can now access the fragment
   and obtain the access token.

4.1.3.  Countermeasures

   The complexity of implementing and managing pattern matching
   correctly obviously causes security issues.  This document therefore
   advises to simplify the required logic and configuration by using
   exact redirect URI matching only.  This means the authorization
   server MUST compare the two URIs using simple string comparison as
   defined in [RFC3986], Section 6.2.1.

   Additional recommendations:

   o  Servers on which callbacks are hosted MUST NOT expose open
      redirectors (see Section 4.9).

   o  Browsers reattach URL fragments to Location redirection URLs only
      if the URL in the Location header does not already contain a
      fragment.  Therefore, servers MAY prevent browsers from
      reattaching fragments to redirection URLs by attaching an
      arbitrary fragment identifier, for example "#_", to URLs in
      Location headers.

   o  Clients SHOULD use the authorization code response type instead of
      response types causing access token issuance at the authorization
      endpoint.  This offers countermeasures against reuse of leaked
      credentials through the exchange process with the authorization




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      server and token replay through sender-constraining of the access
      tokens.

   If the origin and integrity of the authorization request containing
   the redirect URI can be verified, for example when using
   [I-D.ietf-oauth-jwsreq] or [I-D.ietf-oauth-par] with client
   authentication, the authorization server MAY trust the redirect URI
   without further checks.

4.2.  Credential Leakage via Referer Headers

   The contents of the authorization request URI or the authorization
   response URI can unintentionally be disclosed to attackers through
   the Referer HTTP header (see [RFC7231], Section 5.5.2), by leaking
   either from the AS's or the client's web site, respectively.  Most
   importantly, authorization codes or "state" values can be disclosed
   in this way.  Although specified otherwise in [RFC7231],
   Section 5.5.2, the same may happen to access tokens conveyed in URI
   fragments due to browser implementation issues as illustrated by
   Chromium Issue 168213 [bug.chromium].

4.2.1.  Leakage from the OAuth Client

   Leakage from the OAuth client requires that the client, as a result
   of a successful authorization request, renders a page that

   o  contains links to other pages under the attacker's control and a
      user clicks on such a link, or

   o  includes third-party content (advertisements in iframes, images,
      etc.), for example if the page contains user-generated content
      (blog).

   As soon as the browser navigates to the attacker's page or loads the
   third-party content, the attacker receives the authorization response
   URL and can extract "code" or "state" (and potentially "access
   token").

4.2.2.  Leakage from the Authorization Server

   In a similar way, an attacker can learn "state" from the
   authorization request if the authorization endpoint at the
   authorization server contains links or third-party content as above.








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4.2.3.  Consequences

   An attacker that learns a valid code or access token through a
   Referer header can perform the attacks as described in Section 4.1.1,
   Section 4.5, and Section 4.6.  If the attacker learns "state", the
   CSRF protection achieved by using "state" is lost, resulting in CSRF
   attacks as described in [RFC6819], Section 4.4.1.8.

4.2.4.  Countermeasures

   The page rendered as a result of the OAuth authorization response and
   the authorization endpoint SHOULD NOT include third-party resources
   or links to external sites.

   The following measures further reduce the chances of a successful
   attack:

   o  Suppress the Referer header by applying an appropriate Referrer
      Policy [webappsec-referrer-policy] to the document (either as part
      of the "referrer" meta attribute or by setting a Referrer-Policy
      header).  For example, the header "Referrer-Policy: no-referrer"
      in the response completely suppresses the Referer header in all
      requests originating from the resulting document.

   o  Use authorization code instead of response types causing access
      token issuance from the authorization endpoint.

   o  Bind authorization code to a confidential client or PKCE
      challenge.  In this case, the attacker lacks the secret to request
      the code exchange.

   o  As described in [RFC6749], Section 4.1.2, authorization codes MUST
      be invalidated by the AS after their first use at the token
      endpoint.  For example, if an AS invalidated the code after the
      legitimate client redeemed it, the attacker would fail exchanging
      this code later.
      This does not mitigate the attack if the attacker manages to
      exchange the code for a token before the legitimate client does
      so.  Therefore, [RFC6749] further recommends that, when an attempt
      is made to redeem a code twice, the AS SHOULD revoke all tokens
      issued previously based on that code.

   o  The "state" value SHOULD be invalidated by the client after its
      first use at the redirection endpoint.  If this is implemented,
      and an attacker receives a token through the Referer header from
      the client's web site, the "state" was already used, invalidated
      by the client and cannot be used again by the attacker.  (This
      does not help if the "state" leaks from the AS's web site, since



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      then the "state" has not been used at the redirection endpoint at
      the client yet.)

   o  Use the form post response mode instead of a redirect for the
      authorization response (see [oauth-v2-form-post-response-mode]).

4.3.  Credential Leakage via Browser History

   Authorization codes and access tokens can end up in the browser's
   history of visited URLs, enabling the attacks described in the
   following.

4.3.1.  Authorization Code in Browser History

   When a browser navigates to "client.example/
   redirection_endpoint?code=abcd" as a result of a redirect from a
   provider's authorization endpoint, the URL including the
   authorization code may end up in the browser's history.  An attacker
   with access to the device could obtain the code and try to replay it.

   Countermeasures:

   o  Authorization code replay prevention as described in [RFC6819],
      Section 4.4.1.1, and Section 4.5.

   o  Use form post response mode instead of redirect for the
      authorization response (see [oauth-v2-form-post-response-mode]).

4.3.2.  Access Token in Browser History

   An access token may end up in the browser history if a client or a
   web site that already has a token deliberately navigates to a page
   like "provider.com/get_user_profile?access_token=abcdef".  [RFC6750]
   discourages this practice and advises to transfer tokens via a
   header, but in practice web sites often pass access tokens in query
   parameters.

   In case of the implicit grant, a URL like "client.example/
   redirection_endpoint#access_token=abcdef" may also end up in the
   browser history as a result of a redirect from a provider's
   authorization endpoint.

   Countermeasures:

   o  Clients MUST NOT pass access tokens in a URI query parameter in
      the way described in Section 2.3 of [RFC6750].  The authorization
      code grant or alternative OAuth response modes like the form post




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      response mode [oauth-v2-form-post-response-mode] can be used to
      this end.

4.4.  Mix-Up Attacks

   Mix-up is an attack on scenarios where an OAuth client interacts with
   two or more authorization servers and at least one authorization
   server is under the control of the attacker.  This can be the case,
   for example, if the attacker uses dynamic registration to register
   the client at his own authorization server or if an authorization
   server becomes compromised.

   The goal of the attack is to obtain an authorization code or an
   access token for an uncompromised authorization server.  This is
   achieved by tricking the client into sending those credentials to the
   compromised authorization server (the attacker) instead of using them
   at the respective endpoint of the uncompromised authorization/
   resource server.

4.4.1.  Attack Description

   The description here closely follows [arXiv.1601.01229], with
   variants of the attack outlined below.

   Preconditions: For this variant of the attack to work, we assume that

   o  the implicit or authorization code grant are used with multiple AS
      of which one is considered "honest" (H-AS) and one is operated by
      the attacker (A-AS),

   o  the client stores the AS chosen by the user in a session bound to
      the user's browser and uses the same redirection endpoint URI for
      each AS, and

   o  the attacker can intercept and manipulate the first request/
      response pair from a user's browser to the client (in which the
      user selects a certain AS and is then redirected by the client to
      that AS), as in Attacker A2.

   The latter ability can, for example, be the result of a man-in-the-
   middle attack on the user's connection to the client.  Note that an
   attack variant exists that does not require this ability, see below.

   In the following, we assume that the client is registered with H-AS
   (URI: "https://honest.as.example", client ID: "7ZGZldHQ") and with
   A-AS (URI: "https://attacker.example", client ID: "666RVZJTA").

   Attack on the authorization code grant:



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   1.  The user selects to start the grant using H-AS (e.g., by clicking
       on a button at the client's website).

   2.  The attacker intercepts this request and changes the user's
       selection to "A-AS" (see preconditions).

   3.  The client stores in the user's session that the user selected
       "A-AS" and redirects the user to A-AS's authorization endpoint
       with a Location header containing the URL
       "https://attacker.example/
       authorize?response_type=code&client_id=666RVZJTA".

   4.  Now the attacker intercepts this response and changes the
       redirection such that the user is being redirected to H-AS.  The
       attacker also replaces the client ID of the client at A-AS with
       the client's ID at H-AS.  Therefore, the browser receives a
       redirection ("303 See Other") with a Location header pointing to
       "https://honest.as.example/
       authorize?response_type=code&client_id=7ZGZldHQ"

   5.  The user authorizes the client to access her resources at H-AS.
       H-AS issues a code and sends it (via the browser) back to the
       client.

   6.  Since the client still assumes that the code was issued by A-AS,
       it will try to redeem the code at A-AS's token endpoint.

   7.  The attacker therefore obtains code and can either exchange the
       code for an access token (for public clients) or perform an
       authorization code injection attack as described in Section 4.5.

   Variants:

   o  *Mix-Up Without Interception*: A variant of the above attack works
      even if the first request/response pair cannot be intercepted, for
      example, because TLS is used to protect these messages: Here, it
      is assumed that the user wants to start the grant using A-AS (and
      not H-AS, see Attacker A1).  After the client redirected the user
      to the authorization endpoint at A-AS, the attacker immediately
      redirects the user to H-AS (changing the client ID to "7ZGZldHQ").
      Note that a vigilant user might at this point detect that she
      intended to use A-AS instead of H-AS.  The attack now proceeds
      exactly as in Steps 3ff. of the attack description above.

   o  *Implicit Grant*: In the implicit grant, the attacker receives an
      access token instead of the code; the rest of the attack works as
      above.




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   o  *Per-AS Redirect URIs*: If clients use different redirect URIs for
      different ASs, do not store the selected AS in the user's session,
      and ASs do not check the redirect URIs properly, attackers can
      mount an attack called "Cross-Social Network Request Forgery".
      These attacks have been observed in practice.  Refer to
      [oauth_security_jcs_14] for details.

   o  *OpenID Connect*: There are variants that can be used to attack
      OpenID Connect.  In these attacks, the attacker misuses features
      of the OpenID Connect Discovery mechanism or replays access tokens
      or ID Tokens to conduct a Mix-Up Attack.  The attacks are
      described in detail in [arXiv.1704.08539], Appendix A, and
      [arXiv.1508.04324v2], Section 6 ("Malicious Endpoints Attacks").

4.4.2.  Countermeasures

   In scenarios where an OAuth client interacts with multiple
   authorization servers, clients MUST prevent mix-up attacks.

   To this end, clients SHOULD use distinct redirect URIs for each AS
   (with alternatives listed below).  Clients MUST store, for each
   authorization request, the AS they sent the authorization request to
   and bind this information to the user agent.  Clients MUST check that
   the authorization request was received from the correct authorization
   server and ensure that the subsequent token request, if applicable,
   is sent to the same authorization server.

   Unfortunately, distinct redirect URIs per AS do not work for all
   kinds of OAuth clients.  They are effective for web and JavaScript
   apps and for native apps with claimed URLs.  Attacks on native apps
   using custom schemes or redirect URIs on localhost cannot be
   prevented this way.

   If clients cannot use distinct redirect URIs for each AS, the
   following options exist:

   o  Authorization servers can be configured to return an AS
      identitifier ("iss") as a non-standard parameter in the
      authorization response.  This enables complying clients to compare
      this data to the "iss" identifier of the AS it believed it sent
      the user agent to.

   o  In OpenID Connect, if an ID Token is returned in the authorization
      response, it carries client ID and issuer.  It can be used in the
      same way as the "iss" parameter.






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4.5.  Authorization Code Injection

   In an authorization code injection attack, the attacker attempts to
   inject a stolen authorization code into the attacker's own session
   with the client.  The aim is to associate the attacker's session at
   the client with the victim's resources or identity.

   This attack is useful if the attacker cannot exchange the
   authorization code for an access token himself.  Examples include:

   o  The code is bound to a particular confidential client and the
      attacker is unable to obtain the required client credentials to
      redeem the code himself.

   o  The attacker wants to access certain functions in this particular
      client.  As an example, the attacker wants to impersonate his
      victim in a certain app or on a certain web site.

   o  The authorization or resource servers are limited to certain
      networks that the attacker is unable to access directly.

   In the following attack description and discussion, we assume the
   presence of a web (A1) or network attacker (A2).

4.5.1.  Attack Description

   The attack works as follows:

   1.  The attacker obtains an authorization code by performing any of
       the attacks described above.

   2.  He performs a regular OAuth authorization process with the
       legitimate client on his device.

   3.  The attacker injects the stolen authorization code in the
       response of the authorization server to the legitimate client.
       Since this response is passing through the attacker's device, the
       attacker can use any tool that can intercept and manipulate the
       authorization response to this end.  The attacker does not need
       to control the network.

   4.  The legitimate client sends the code to the authorization
       server's token endpoint, along with the client's client ID,
       client secret and actual "redirect_uri".

   5.  The authorization server checks the client secret, whether the
       code was issued to the particular client, and whether the actual




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       redirect URI matches the "redirect_uri" parameter (see
       [RFC6749]).

   6.  All checks succeed and the authorization server issues access and
       other tokens to the client.  The attacker has now associated his
       session with the legitimate client with the victim's resources
       and/or identity.

4.5.2.  Discussion

   Obviously, the check in step (5.) will fail if the code was issued to
   another client ID, e.g., a client set up by the attacker.  The check
   will also fail if the authorization code was already redeemed by the
   legitimate user and was one-time use only.

   An attempt to inject a code obtained via a manipulated redirect URI
   should also be detected if the authorization server stored the
   complete redirect URI used in the authorization request and compares
   it with the "redirect_uri" parameter.

   [RFC6749], Section 4.1.3, requires the AS to "... ensure that the
   "redirect_uri" parameter is present if the "redirect_uri" parameter
   was included in the initial authorization request as described in
   Section 4.1.1, and if included ensure that their values are
   identical.".  In the attack scenario described above, the legitimate
   client would use the correct redirect URI it always uses for
   authorization requests.  But this URI would not match the tampered
   redirect URI used by the attacker (otherwise, the redirect would not
   land at the attackers page).  So the authorization server would
   detect the attack and refuse to exchange the code.

   Note: this check could also detect attempts to inject an
   authorization code which had been obtained from another instance of
   the same client on another device, if certain conditions are
   fulfilled:

   o  the redirect URI itself needs to contain a nonce or another kind
      of one-time use, secret data and

   o  the client has bound this data to this particular instance of the
      client.

   But this approach conflicts with the idea to enforce exact redirect
   URI matching at the authorization endpoint.  Moreover, it has been
   observed that providers very often ignore the "redirect_uri" check
   requirement at this stage, maybe because it doesn't seem to be
   security-critical from reading the specification.




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   Other providers just pattern match the "redirect_uri" parameter
   against the registered redirect URI pattern.  This saves the
   authorization server from storing the link between the actual
   redirect URI and the respective authorization code for every
   transaction.  But this kind of check obviously does not fulfill the
   intent of the specification, since the tampered redirect URI is not
   considered.  So any attempt to inject an authorization code obtained
   using the "client_id" of a legitimate client or by utilizing the
   legitimate client on another device will not be detected in the
   respective deployments.

   It is also assumed that the requirements defined in [RFC6749],
   Section 4.1.3, increase client implementation complexity as clients
   need to store or re-construct the correct redirect URI for the call
   to the token endpoint.

   This document therefore recommends to instead bind every
   authorization code to a certain client instance on a certain device
   (or in a certain user agent) in the context of a certain transaction
   using one of the mechanisms described next.

4.5.3.  Countermeasures

   There are two good technical solutions to achieve this goal:

   o  *PKCE*: The PKCE parameter "code_challenge" along with the
      corresponding "code_verifier" as specified in [RFC7636] can be
      used as a countermeasure.  In contrast to its original intention,
      the verifier check fails although the client uses its correct
      verifier but the code is associated with a challenge that does not
      match.  PKCE is a deployed OAuth feature, although its original
      intended use was solely focused on securing native apps, not the
      broader use recommended by this document.

   o  *Nonce*: OpenID Connect's existing "nonce" parameter can be used
      for the same purpose.  The "nonce" value is one-time use and
      created by the client.  The client is supposed to bind it to the
      user agent session and sends it with the initial request to the
      OpenID Provider (OP).  The OP binds "nonce" to the authorization
      code and attests this binding in the ID Token, which is issued as
      part of the code exchange at the token endpoint.  If an attacker
      injected an authorization code in the authorization response, the
      nonce value in the client session and the nonce value in the ID
      token will not match and the attack is detected.  The assumption
      is that an attacker cannot get hold of the user agent state on the
      victim's device, where he has stolen the respective authorization
      code.




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   Other solutions, like binding "state" to the code, using token
   binding for the code, or per-instance client credentials are
   conceivable, but lack support and bring new security requirements.

   PKCE is the most obvious solution for OAuth clients as it is
   available today (originally intended for OAuth native apps) whereas
   "nonce" is appropriate for OpenID Connect clients.

4.5.4.  Limitations

   An attacker can circumvent the countermeasures described above if he
   can modify the "nonce" or "code_challenge" values that are used in
   the victim's authorization request.  The attacker can modify these
   values to be the same ones as those chosen by the client in his own
   session in Step 2 of the attack above.  (This requires that the
   victim's session with the client begins after the attacker started
   his session with the client.)  If the attacker is then able to
   capture the authorization code from the victim, the attacker will be
   able to inject the stolen code in Step 3 even if PKCE or "nonce" are
   used.

   This attack is complex and requires a close interaction between the
   attacker and the victim's session.  Nonetheless, measures to prevent
   attackers from reading the contents of the authorization response
   still need to be taken, as described in Section 4.1, Section 4.2,
   Section 4.3, Section 4.4, and Section 4.9.

4.6.  Access Token Injection

   In an access token injection attack, the attacker attempts to inject
   a stolen access token into a legitimate client (that is not under the
   attacker's control).  This will typically happen if the attacker
   wants to utilize a leaked access token to impersonate a user in a
   certain client.

   To conduct the attack, the attacker starts an OAuth flow with the
   client using the implicit grant and modifies the authorization
   response by replacing the access token issued by the authorization
   server or directly makes up an authorization server response
   including the leaked access token.  Since the response includes the
   "state" value generated by the client for this particular
   transaction, the client does not treat the response as a CSRF attack
   and uses the access token injected by the attacker.








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4.6.1.  Countermeasures

   There is no way to detect such an injection attack on the OAuth
   protocol level, since the token is issued without any binding to the
   transaction or the particular user agent.

   The recommendation is therefore to use the authorization code grant
   type instead of relying on response types issuing acess tokens at the
   authorization endpoint.  Authorization code injection can be detected
   using one of the countermeasures discussed in Section 4.5.

4.7.  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).

4.7.1.  Countermeasures

   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
   PKCE or the OpenID Connect "nonce" value.

   When using PKCE instead of "state" or "nonce" for CSRF protection, it
   is important to note that:

   o  Clients MUST ensure that the AS supports PKCE before using PKCE
      for CSRF protection.  If an authorization server does not support
      PKCE, "state" or "nonce" MUST be used for CSRF protection.

   o  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 support for PKCE
   either via AS metadata according to [RFC8414] or provide a
   deployment-specific way to ensure or determine PKCE support.

4.8.  Access Token Leakage at the Resource Server

   Access tokens can leak from a resource server under certain
   circumstances.





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4.8.1.  Access Token Phishing by Counterfeit Resource Server

   An attacker may setup his own resource server and trick a client into
   sending access tokens to it that are valid for other resource servers
   (see Attackers A1 and A5).  If the client sends a valid access token
   to this counterfeit resource server, the attacker in turn may use
   that token to access other services on behalf of the resource owner.

   This attack assumes the client is not bound to one specific resource
   server (and its URL) at development time, but client instances are
   provided with the resource server URL at runtime.  This kind of late
   binding is typical in situations where the client uses a service
   implementing a standardized API (e.g., for e-Mail, calendar, health,
   or banking) and where the client is configured by a user or
   administrator for a service which this user or company uses.

4.8.1.1.  Countermeasures

   There are several potential mitigation strategies, which will be
   discussed in the following sections.

4.8.1.1.1.  Metadata

   An authorization server could provide the client with additional
   information about the location where it is safe to use its access
   tokens.

   In the simplest form, this would require the AS to publish a list of
   its known resource servers, illustrated in the following example
   using a non-standard metadata parameter "resource_servers":

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "issuer":"https://server.somesite.example",
     "authorization_endpoint":
       "https://server.somesite.example/authorize",
     "resource_servers":[
       "email.somesite.example",
       "storage.somesite.example",
       "video.somesite.example"
     ]
     ...
   }






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   The AS could also return the URL(s) an access token is good for in
   the token response, illustrated by the example and non-standard
   return parameter "access_token_resource_server":

   HTTP/1.1 200 OK
   Content-Type: application/json;charset=UTF-8
   Cache-Control: no-store
   Pragma: no-cache

   {
     "access_token":"2YotnFZFEjr1zCsicMWpAA",
     "access_token_resource_server":
       "https://hostedresource.somesite.example/path1",
   ...
   }

   This mitigation strategy would rely on the client to enforce the
   security policy and to only send access tokens to legitimate
   destinations.  Results of OAuth related security research (see for
   example [oauth_security_ubc] and [oauth_security_cmu]) indicate a
   large portion of client implementations do not or fail to properly
   implement security controls, like "state" checks.  So relying on
   clients to prevent access token phishing is likely to fail as well.
   Moreover given the ratio of clients to authorization and resource
   servers, it is considered the more viable approach to move as much as
   possible security-related logic to those entities.  Clearly, the
   client has to contribute to the overall security.  But there are
   alternative countermeasures, as described in the next sections, which
   provide a better balance between the involved parties.

4.8.1.1.2.  Sender-Constrained Access Tokens

   As the name suggests, sender-constrained access token scope 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 token at a resource server.

   A typical flow looks like this:

   1.  The authorization server associates data with the access token
       that binds this particular token to a certain client.  The
       binding can utilize the client identity, but in most cases the AS
       utilizes key material (or data derived from the key material)
       known to the client.

   2.  This key material must be distributed somehow.  Either the key
       material already exists before the AS creates the binding or the
       AS creates ephemeral keys.  The way pre-existing key material is



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       distributed varies among the different approaches.  For example,
       X.509 Certificates can be used in which case the distribution
       happens explicitly during the enrollment process.  Or the key
       material is created and distributed at the TLS layer, in which
       case it might automatically happen during the setup of a TLS
       connection.

   3.  The RS must implement the actual proof of possession check.  This
       is typically done on the application level, often tied to
       specific material provided by transport layer (e.g., TLS).  The
       RS must also ensure that replay of the proof of possession is not
       possible.

   There exist several proposals to demonstrate the proof of possession
   in the scope of the OAuth working group:

   o  *OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
      Access Tokens* ([RFC8705]): The approach as specified in this
      document allows the use of mutual TLS (mTLS) for both client
      authentication and sender-constrained access tokens.  For the
      purpose of sender-constrained access tokens, the client is
      identified towards the resource server by the fingerprint of its
      public key.  During processing of an access token request, the
      authorization server obtains the client's public key from the TLS
      stack and associates its fingerprint with the respective access
      tokens.  The resource server in the same way obtains the public
      key from the TLS stack and compares its fingerprint with the
      fingerprint associated with the access token.

   o  *DPoP* ([I-D.fett-oauth-dpop]): DPoP (Demonstration of Proof-of-
      Possession at the Application Layer) outlines an application-level
      sender-constraining for access and refresh tokens that can be used
      in cases where neither mTLS nor OAuth Token Binding (see below)
      are available.  It uses proof-of-possession based on a public/
      private key pair and application-level signing.  DPoP can be used
      with public clients and, in case of confidential clients, can be
      combined with any client authentication method.

   o  *OAuth Token Binding* ([I-D.ietf-oauth-token-binding]): In this
      approach, an access token is, via the token binding ID, bound to
      key material representing a long term association between a client
      and a certain TLS host.  Negotiation of the key material and proof
      of possession in the context of a TLS handshake is taken care of
      by the TLS stack.  The client needs to determine the token binding
      ID of the target resource server and pass this data to the access
      token request.  The authorization server then associates the
      access token with this ID.  The resource server checks on every
      invocation that the token binding ID of the active TLS connection



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      and the token binding ID of associated with the access token
      match.  Since all crypto-related functions are covered by the TLS
      stack, this approach is very client developer friendly.  As a
      prerequisite, token binding as described in [RFC8473] (including
      federated token bindings) must be supported on all ends (client,
      authorization server, resource server).

   o  *Signed HTTP Requests* ([I-D.ietf-oauth-signed-http-request]):
      This approach utilizes [I-D.ietf-oauth-pop-key-distribution] and
      represents the elements of the signature in a JSON object.  The
      signature is built using JWS.  The mechanism has built-in support
      for signing of HTTP method, query parameters and headers.  It also
      incorporates a timestamp as basis for replay prevention.

   o  *JWT Pop Tokens* ([I-D.sakimura-oauth-jpop]): This draft describes
      different ways to constrain access token usage, namely TLS or
      request signing.  Note: Since the authors of this draft
      contributed the TLS-related proposal to [RFC8705], this document
      only considers the request signing part.  For request signing, the
      draft utilizes [I-D.ietf-oauth-pop-key-distribution] and
      [RFC7800].  The signature data is represented in a JWT and JWS is
      used for signing.  Replay prevention is provided by building the
      signature over a server-provided nonce, client-provided nonce and
      a nonce counter.

   At the time of writing, OAuth Mutual TLS is the most widely
   implemented and the only standardized sender-constraining method.
   The use of OAuth Mutual TLS therefore is RECOMMENDED.

   Note that the security of sender-constrained tokens is undermined
   when an attacker gets access to the token and the key material.  This
   is in particular the case for corrupted client software and cross-
   site scripting attacks (when the client is running in the browser).
   If the key material is protected in a hardware or software security
   module or only indirectly accessible (like in a TLS stack), sender-
   constrained tokens at least protect against a use of the token when
   the client is offline, i.e., when the security module or interface is
   not available to the attacker.  This applies to access tokens as well
   as to refresh tokens (see Section 4.12).

4.8.1.1.3.  Audience Restricted Access Tokens

   Audience restriction essentially restricts access tokens to a
   particular resource server.  The authorization server associates the
   access token with the particular resource server and the resource
   server SHOULD verify the intended audience.  If the access token
   fails the intended audience validation, the resource server must
   refuse to serve the respective request.



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   In general, audience restrictions limit the impact of token leakage.
   In the case of a counterfeit resource server, it may (as described
   below) also prevent abuse of the phished access token at the
   legitimate resource server.

   The audience can be expressed using logical names or physical
   addresses (like URLs).  In order to prevent phishing, it is necessary
   to use the actual URL the client will send requests to.  In the
   phishing case, this URL will point to the counterfeit resource
   server.  If the attacker tries to use the access token at the
   legitimate resource server (which has a different URL), the resource
   server will detect the mismatch (wrong audience) and refuse to serve
   the request.

   In deployments where the authorization server knows the URLs of all
   resource servers, the authorization server may just refuse to issue
   access tokens for unknown resource server URLs.

   The client SHOULD tell the authorization server the intended resource
   server.  The proposed mechanism [I-D.ietf-oauth-resource-indicators]
   could be used or by encoding the information in the scope value.

   Instead of the URL, it is also possible to utilize the fingerprint of
   the resource server's X.509 certificate as audience value.  This
   variant would also allow to detect an attempt to spoof the legitimate
   resource server's URL by using a valid TLS certificate obtained from
   a different CA.  It might also be considered a privacy benefit to
   hide the resource server URL from the authorization server.

   Audience restriction may seem easier to use since it does not require
   any crypto on the client-side.  Still, since every access token is
   bound to a specific resource server, the client also needs to obtain
   a single RS-specific access token when accessing several resource
   servers.  (Resource indicators, as specified in
   [I-D.ietf-oauth-resource-indicators], can help to achieve this.)
   [I-D.ietf-oauth-token-binding] has the same property since different
   token binding ids must be associated with the access token.  Using
   [RFC8705], on the other hand, allows a client to use the access token
   at multiple resource servers.

   It shall be noted that audience restrictions, or generally speaking
   an indication by the client to the authorization server where it
   wants to use the access token, has additional benefits beyond the
   scope of token leakage prevention.  It allows the authorization
   server to create different access token whose format and content is
   specifically minted for the respective server.  This has huge
   functional and privacy advantages in deployments using structured
   access tokens.



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4.8.2.  Compromised Resource Server

   An attacker may compromise a resource server to gain access to the
   resources of the respective deployment.  Such a compromise may range
   from partial access to the system, e.g., its log files, to full
   control of the respective server.

   If the attacker were able to gain full control, including shell
   access, all controls can be circumvented and all resources be
   accessed.  The attacker would also be able to obtain other access
   tokens held on the compromised system that would potentially be valid
   to access other resource servers.

   Preventing server breaches by hardening and monitoring server systems
   is considered a standard operational procedure and, therefore, out of
   the scope of this document.  This section focuses on the impact of
   OAuth-related breaches and the replaying of captured access tokens.

   The following measures should be taken into account by implementers
   in order to cope with access token replay by malicious actors:

   o  Sender-constrained access tokens as described in Section 4.8.1.1.2
      SHOULD be used to prevent the attacker from replaying the access
      tokens on other resource servers.  Depending on the severity of
      the penetration, sender-constrained access tokens will also
      prevent replay on the compromised system.

   o  Audience restriction as described in Section 4.8.1.1.3 SHOULD be
      used to prevent replay of captured access tokens on other resource
      servers.

   o  The resource server MUST treat access tokens like any other
      credentials.  It is considered good practice to not log them and
      not store them in plain text.

   The first and second recommendation also apply to other scenarios
   where access tokens leak (see Attacker A5).

4.9.  Open Redirection

   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.








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4.9.1.  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
   Section 4.1.2.  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].

4.9.2.  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.

   [RFC6749], 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.

4.10.  307 Redirect

   At the authorization endpoint, a typical protocol flow is that the AS
   prompts the user to enter her 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 redirection endpoint.





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   In [RFC6749], the HTTP status code 302 is used for this purpose, but
   "any other method available via the user-agent to accomplish this
   redirection is allowed".  When the status code 307 is used for
   redirection instead, the user agent will 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
   agents can 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.)

   AS which redirect a request that potentially contains user
   credentials therefore 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".

4.11.  TLS Terminating Reverse Proxies

   A common deployment architecture for HTTP applications is to hide the
   application server behind a reverse proxy that terminates the TLS
   connection and dispatches the incoming requests to the respective
   application server nodes.

   This section highlights some attack angles of this deployment
   architecture with relevance to OAuth and gives recommendations for
   security controls.

   In some situations, the reverse proxy needs to pass security-related
   data to the upstream application servers for further processing.
   Examples include the IP address of the request originator, token
   binding ids, and authenticated TLS client certificates.  This data is
   usually passed in custom HTTP headers added to the upstream request.

   If the reverse proxy would pass through any header sent from the
   outside, an attacker could try to directly send the faked header
   values through the proxy to the application server in order to



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   circumvent security controls that way.  For example, it is standard
   practice of reverse proxies to accept "X-Forwarded-For" headers and
   just add the origin of the inbound request (making it a list).
   Depending on the logic performed in the application server, the
   attacker could simply add a whitelisted IP address to the header and
   render a IP whitelist useless.

   A reverse proxy must therefore sanitize any inbound requests to
   ensure the authenticity and integrity of all header values relevant
   for the security of the application servers.

   If an attacker was able to get access to the internal network between
   proxy and application server, the attacker could also try to
   circumvent security controls in place.  It is, therefore, essential
   to ensure the authenticity of the communicating entities.
   Furthermore, the communication link between reverse proxy and
   application server must be protected against eavesdropping,
   injection, and replay of messages.

4.12.  Refresh Token Protection

   Refresh tokens are a convenient and user-friendly way to obtain new
   access tokens after the expiration of access tokens.  Refresh tokens
   also add to the security of OAuth since they allow the authorization
   server to issue access tokens with a short lifetime and reduced scope
   thus reducing the potential impact of access token leakage.

4.12.1.  Discussion

   Refresh tokens are an attractive target for attackers since they
   represent the overall grant a resource owner delegated to a certain
   client.  If an attacker is able to exfiltrate and successfully replay
   a refresh token, the attacker will be able to mint access tokens and
   use them to access resource servers on behalf of the resource owner.

   [RFC6749] already provides a robust baseline protection by requiring

   o  confidentiality of the refresh tokens in transit and storage,

   o  the transmission of refresh tokens over TLS-protected connections
      between authorization server and client,

   o  the authorization server to maintain and check the binding of a
      refresh token to a certain client (i.e., "client_id"),

   o  authentication of this client during token refresh, if possible,
      and




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   o  that refresh tokens cannot be generated, modified, or guessed.

   [RFC6749] also lays the foundation for further (implementation
   specific) security measures, such as refresh token expiration and
   revocation as well as refresh token rotation by defining respective
   error codes and response behavior.

   This specification gives recommendations beyond the scope of
   [RFC6749] and clarifications.

4.12.2.  Recommendations

   Authorization servers SHOULD determine, based on a risk assessment,
   whether to issue refresh tokens to a certain client.  If the
   authorization server decides not to issue refresh tokens, the client
   MAY refresh access tokens by utilizing other grant types, such as the
   authorization code grant type.  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 the legitimate client and
   reduce the impact of refresh token leakage.

   Authorization server MUST utilize one of these methods to detect
   refresh token replay by malicious actors for public clients:

   o  *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] or [RFC8705].

   o  *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



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      need to be revoked.  Authorization servers MUST ensure the
      integrity of the refresh token value in this case, for example,
      using signatures.

   Authorization servers MAY revoke refresh tokens automatically in case
   of a security event, such as:

   o  password change

   o  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 fresh
   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).

4.13.  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.

4.13.1.  Countermeasures

   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.

4.14.  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.




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   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.

5.  Acknowledgements

   We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
   Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen,
   Vittorio Bertocci, David Waite, Nov Matake, Tomek Stojecki, Dominick
   Baier, Neil Madden, William Dennis, Dick Hardt, Petteri Stenius,
   Annabelle Richard Backman, Aaron Parecki, George Fletscher, Brian
   Campbell, Konstantin Lapine, Tim Wuertele, Guido Schmitz, Hans
   Zandbelt, Jared Jennings, Michael Peck, Pedram Hosseyni, Michael B.
   Jones, and Travis Spencer for their valuable feedback.




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6.  IANA Considerations

   This draft includes no request to IANA.

7.  Security Considerations

   All relevant security considerations have been given in the
   functional specification.

8.  References

8.1.  Normative References

   [oauth-v2-form-post-response-mode]
              Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
              Mode", April 2015, <http://openid.net/specs/oauth-v2-form-
              post-response-mode-1_0.html>.

   [OpenID]   Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", Nov 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [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>.

   [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>.

   [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>.

   [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>.





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   [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>.

   [RFC8418]  Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
              Agreement Algorithm with X25519 and X448 in the
              Cryptographic Message Syntax (CMS)", RFC 8418,
              DOI 10.17487/RFC8418, August 2018,
              <https://www.rfc-editor.org/info/rfc8418>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

8.2.  Informative References

   [arXiv.1508.04324v2]
              Mladenov, V., Mainka, C., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", January 2016,
              <http://arxiv.org/abs/1508.04324v2/>.

   [arXiv.1601.01229]
              Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0", January 2016,
              <http://arxiv.org/abs/1601.01229/>.

   [arXiv.1704.08539]
              Fett, D., Kuesters, R., and G. Schmitz, "The Web SSO
              Standard OpenID Connect: In-Depth Formal Security Analysis
              and Security Guidelines", April 2017,
              <http://arxiv.org/abs/1704.08539/>.

   [arXiv.1901.11520]
              Fett, D., Hosseyni, P., and R. Kuesters, "An Extensive
              Formal Security Analysis of the OpenID Financial-grade
              API", January 2019, <http://arxiv.org/abs/1901.11520/>.



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   [bug.chromium]
              "Referer header includes URL fragment when opening link
              using New Tab",
              <https://bugs.chromium.org/p/chromium/issues/
              detail?id=168213/>.

   [CSP-2]    West, M., Barth, A., and D. Veditz, "Content Security
              Policy Level 2", July 2015, <https://www.w3.org/TR/CSP2>.

   [I-D.bradley-oauth-jwt-encoded-state]
              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", draft-
              bradley-oauth-jwt-encoded-state-09 (work in progress),
              November 2018.

   [I-D.fett-oauth-dpop]
              Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstration of
              Proof-of-Possession at the Application Layer (DPoP)",
              draft-fett-oauth-dpop-03 (work in progress), October 2019.

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)",
              draft-ietf-oauth-jwsreq-20 (work in progress), October
              2019.

   [I-D.ietf-oauth-par]
              Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
              and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
              draft-ietf-oauth-par-00 (work in progress), December 2019.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M.
              Meszaros, "OAuth 2.0 Proof-of-Possession: Authorization
              Server to Client Key Distribution", draft-ietf-oauth-pop-
              key-distribution-07 (work in progress), March 2019.

   [I-D.ietf-oauth-rar]
              Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", draft-ietf-oauth-rar-00
              (work in progress), January 2020.

   [I-D.ietf-oauth-resource-indicators]
              Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", draft-ietf-oauth-resource-
              indicators-08 (work in progress), September 2019.




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   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", draft-ietf-oauth-signed-
              http-request-03 (work in progress), August 2016.

   [I-D.ietf-oauth-token-binding]
              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-08 (work in progress), October 2018.

   [I-D.sakimura-oauth-jpop]
              Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
              Authorization Framework: JWT Pop Token Usage", draft-
              sakimura-oauth-jpop-05 (work in progress), July 2019.

   [oauth_security_cmu]
              Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
              Tague, "OAuth Demystified for Mobile Application
              Developers", November 2014,
              <http://css.csail.mit.edu/6.858/2012/readings/oauth-
              sso.pdf>.

   [oauth_security_jcs_14]
              Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
              Maffeis, "Discovering concrete attacks on website
              authorization by formal analysis", April 2014,
              <https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.

   [oauth_security_ubc]
              Sun, S. and K. Beznosov, "The Devil is in the
              (Implementation) Details: An Empirical Analysis of OAuth
              SSO Systems", October 2012,
              <http://passwordresearch.com/papers/paper267.html>.

   [owasp_redir]
              "OWASP Cheat Sheet Series - Unvalidated Redirects and
              Forwards",
              <https://cheatsheetseries.owasp.org/cheatsheets/
              Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.

   [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>.







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   [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>.

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/info/rfc7800>.

   [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>.

   [RFC8473]  Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
              J. Hodges, "Token Binding over HTTP", RFC 8473,
              DOI 10.17487/RFC8473, October 2018,
              <https://www.rfc-editor.org/info/rfc8473>.

   [subdomaintakeover]
              Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
              to Us: Understanding the Security Threats of Dangling DNS
              Records", October 2016,
              <https://www.eecis.udel.edu/~hnw/paper/ccs16a.pdf>.

   [webappsec-referrer-policy]
              Eisinger, J. and E. Stark, "Referrer Policy", April 2017,
              <https://w3c.github.io/webappsec-referrer-policy>.

   [webauthn]
              Balfanz, D., Czeskis, A., Hodges, J., Jones, J., Jones,
              M., Kumar, A., Liao, A., Lindemann, R., and E. Lundberg,
              "Web Authentication: An API for accessing Public Key
              Credentials Level 1", March 2019,
              <https://www.w3.org/TR/2019/REC-webauthn-1-20190304/>.

   [webcrypto]
              Watson, M., "Web Cryptography API", January 2017,
              <https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -15

   o  Added info about using CSP to prevent clickjacking




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   -14

   o  Changes from WGLC feedback

   o  Editorial changes

   o  AS MUST announce PKCE support either in metadata or using
      deployment-specific ways (before: SHOULD)

   -13

   o  Discourage use of Resource Owner Password Credentials Grant

   o  Added text on client impersonating resource owner

   o  Recommend asymmetric methods for client authentication

   o  Encourage use of PKCE mode "S256"

   o  PKCE may replace state for CSRF protection

   o  AS SHOULD publish PKCE support

   o  Cleaned up discussion on auth code injection

   o  AS MUST support PKCE

   -12

   o  Added updated attacker model

   -11

   o  Adapted section 2.1.2 to outcome of consensus call

   o  more text on refresh token inactivity and implementation note on
      refresh token replay detection via refresh token rotation

   -10

   o  incorporated feedback by Joseph Heenan

   o  changed occurrences of SHALL to MUST

   o  added text on lack of token/cert binding support tokens issued in
      the authorization response as justification to not recommend
      issuing tokens there at all




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   o  added requirement to authenticate clients during code exchange
      (PKCE or client credential) to 2.1.1.

   o  added section on refresh tokens

   o  editorial enhancements to 2.1.2 based on feedback

   -09

   o  changed text to recommend not to use implicit but code

   o  added section on access token injection

   o  reworked sections 3.1 through 3.3 to be more specific on implicit
      grant issues

   -08

   o  added recommendations re implicit and token injection

   o  uppercased key words in Section 2 according to RFC 2119

   -07

   o  incorporated findings of Doug McDorman

   o  added section on HTTP status codes for redirects

   o  added new section on access token privilege restriction based on
      comments from Johan Peeters

   -06

   o  reworked section 3.8.1

   o  incorporated Phil Hunt's feedback

   o  reworked section on mix-up

   o  extended section on code leakage via referrer header to also cover
      state leakage

   o  added Daniel Fett as author

   o  replaced text intended to inform WG discussion by recommendations
      to implementors

   o  modified example URLs to conform to RFC 2606



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   -05

   o  Completed sections on code leakage via referrer header, attacks in
      browser, mix-up, and CSRF

   o  Reworked Code Injection Section

   o  Added reference to OpenID Connect spec

   o  removed refresh token leakage as respective considerations have
      been given in section 10.4 of RFC 6749

   o  first version on open redirection

   o  incorporated Christian Mainka's review feedback

   -04

   o  Restructured document for better readability

   o  Added best practices on Token Leakage prevention

   -03

   o  Added section on Access Token Leakage at Resource Server

   o  incorporated Brian Campbell's findings

   -02

   o  Folded Mix up and Access Token leakage through a bad AS into new
      section for dynamic OAuth threats

   o  reworked dynamic OAuth section

   -01

   o  Added references to mitigation methods for token leakage

   o  Added reference to Token Binding for Authorization Code

   o  incorporated feedback of Phil Hunt

   o  fixed numbering issue in attack descriptions in section 2

   -00 (WG document)

   o  turned the ID into a WG document and a BCP



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   o  Added federated app login as topic in Other Topics

Authors' Addresses

   Torsten Lodderstedt
   yes.com

   Email: torsten@lodderstedt.net


   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com


   Andrey Labunets

   Email: isciurus@gmail.com


   Daniel Fett
   yes.com

   Email: mail@danielfett.de


























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