OAuth Working Group B. Campbell
Internet-Draft Ping Identity
Intended status: Standards Track J. Bradley
Expires: April 22, 2019 Yubico
H. Tschofenig
Arm Limited
October 19, 2018

Resource Indicators for OAuth 2.0


An extension to the OAuth 2.0 Authorization Framework defining request parameters that enable a client to explicitly signal to an authorization server about the location of the protected resource(s) to which it is requesting access.

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Table of Contents

1. Introduction

Several years of deployment and implementation experience with The OAuth 2.0 Authorization Framework has uncovered a need, in some circumstances, for the client to explicitly signal to the authorization server where it intends to use the access token it is requesting.

Knowing the protected resource (a.k.a. resource server, application, API, etc.) that will process the access token enables the authorization server to construct the token as necessary for that entity. Properly encrypting the token (or content within the token) to a particular resource, for example, requires knowing which resource will receive and decrypt the token. Furthermore, various resources oftentimes have different requirements with respect to the data contained in, or referenced by, the token and knowing the resource where the client intends to use the token allows the the authorization server to mint the token accordingly.

Specific knowledge of the intended recipient(s) of the access token also helps facilitate improved security characteristics of the token itself. Bearer tokens, currently the most commonly utilized type of OAuth access token, allow any party in possession of a token to get access to the associated resources. To prevent misuse, several important security assumptions must hold, one of which is that an access token must only be valid for use at a specific protected resource and for a specific scope of access. Section 5.2 of [RFC6750], for example, prescribes including the token's intended recipients within the token to prevent token redirect. When the authorization server is informed of the resource that will process the access token, it can restrict the intended audience of that token to the given resource such that the token cannot be used successfully at other resources.

OAuth scope, from Section 3.3 of [RFC6749], is sometimes overloaded to convey the location or identity of the protected resource, however, doing so isn't always feasible or desirable. Scope is typically about what access is being requested rather than where that access will be redeemed (e.g. email, admin:org, user_photos, channels:read, and channels:write are a small sample of scope values in use in the wild that convey only the type of access and not the location).

In some circumstances and for some deployments, a means for the client to signal to the authorization server where it intends to use the access token it's requesting is important and useful. A number of implementations and deployments of OAuth 2.0 have already employed proprietary parameters toward that end. Going forward, this specification aspires to provide a standardized and interoperable alternative to the proprietary approaches.

1.1. Requirements Notation and Conventions

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.

1.2. Terminology

This specification uses the terms "access token", "refresh token", "authorization server", "resource server", "authorization endpoint", "authorization request", "authorization response", "token endpoint", "grant type", "access token request", "access token response", and "client" defined by The OAuth 2.0 Authorization Framework.

2. Resource Parameter

In requests to the authorization server, a client MAY indicate the protected resource (a.k.a. resource server, application, API, etc.) to which it is requesting access by including the following parameter in the request.


Indicates the location of the target service or resource where access is being requested. Its value MUST be an absolute URI, as specified by Section 4.3 of [RFC3986], which MAY include a query component but MUST NOT include a fragment component. Multiple resource parameters MAY be used to indicate that the requested token is intended to be used at multiple resources.

The parameter value indicates the location of a protected resource, typically as an https URL, where the client is requesting access. This enables the authorization server to apply policy as appropriate for the resource, such as determining the type and content of tokens to be issued, if and how tokens are encrypted, and applying appropriate audience restrictions.

The client SHOULD provide the most specific URI that it can for the complete API or set of resources it intends to access. In practice a client will know a base URI for the application or resource that it interacts with, which is appropriate to use as the value of the resource parameter. The client SHOULD use the base URI of the API as the resource parameter value unless specific knowledge of the resource dictates otherwise. For example, the value https://api.example.com/ would be used for a resource that is the exclusive application on that host, however, if the resource is one of many applications on that host, something like https://api.example.com/app/ would be used as a more specific value. Another example, for an API like SCIM that has multiple endpoints such as https://apps.example.com/scim/Users, https://apps.example.com/scim/Groups, and https://apps.example.com/scim/Schemas The client would use https://apps.example.com/scim/ as the resource so that the issued access token is valid for all the endpoints of the SCIM API.

The following error code is provided for an authorization server to indicate problems with the requested resource(s) in response to an authorization request or access token request. And can also be used to inform the client that it has requested an invalid combination of resource and scope.


The requested resource is invalid, unknown, or malformed.

The authorization server SHOULD audience restrict issued access tokens to the resource(s) indicated by the resource parameter. Audience restrictions can be communicated in JSON Web Tokens with the aud claim and the top-level member of the same name provides the audience restriction information in a Token Introspection response. The authorization server may use the exact resource value as the audience or it may map from that value to a more general URI or abstract identifier for the given resource.

2.1. Authorization Request

When the resource parameter is used in an authorization request to the authorization endpoint, it indicates the location of the protected resource(s) to which access is being requested. When an access token will be returned directly from the authorization endpoint via the implicit flow (Section 4.2 of OAuth 2.0), the requested resource is applicable to that access token. In the code flow (Section 4.1 of OAuth 2.0) where an an intermediate representation of the authorization grant (the authorization code) is returned from the authorization endpoint, the requested resource is applicable to the full authorization grant.

For authorization requests sent as a JWTs, such as when using JWT Secured Authorization Request, a single resource parameter value is represented as a JSON string while multiple values are represented as an array of strings.

If the client omits the resource parameter when requesting authorization, the authorization server MAY process the request with no specific resource or by using a pre-defined default resource value. Alternatively, the authorization server MAY require clients to specify the resource(s) they intend to access and MAY fail requests that omit the parameter with an invalid_target error. The authorization server might use this data to inform the user about the resources the client is going to access on her behalf, to meet policy decision (e.g. refuse the request due to unknown resources), and determine the set of resources that can be used in subsequent access token requests.

If the authorization server fails to parse the provided value(s) or does not consider the resource(s) acceptable, it should reject the request with an an error response using the error code invalid_target as the value of the error parameter and can provide additional information regarding the reasons for the error using the error_description and/or error_uri parameters.

  GET /as/authorization.oauth2?response_type=token
     &resource=https%3A%2F%2Fapi.example.com%2Fapp%2F HTTP/1.1
  Host: authorization-server.example.com

Figure 1: Implicit Flow Authorization Request

An example of an authorization request where the client tells the authorization server that it wants an access token for use at https://api.example.com/app/ is shown in Figure 1 below (extra line breaks and indentation are for display purposes only).

  GET /as/authorization.oauth2?response_type=code
     &resource=https%3A%2F%2Fcontacts.example.com%2F HTTP/1.1
  Host: authorization-server.example.com

Figure 2: Code Flow Authorization Request

Below in Figure 2 is an example of an authorization request using the code response type where the the client is requesting access to the resource owner's contacts and calendar data at https://cal.example.com/ and https://contacts.example.com/ (extra line breaks and indentation are for display purposes only).

2.2. Access Token Request

When the resource parameter is used on an access token request made to the token endpoint, for all grant types, it indicates the location of the target service or protected resource where the client intends to use the requested access token.

The resource value(s) that are acceptable to an authorization server in fulfilling an access token request are at its sole discretion based on local policy or configuration. In the case of a refresh_token or authorization_code grant type request, such policy may limit the acceptable resources to those that were originally granted by the resource owner or a subset thereof. In the authorization_code case where the requested resources are a subset of the set of resources originally granted, the authorization server will issue an access token based on that subset of requested resources while any refresh token that is returned is bound to the full original grant.

When requesting a token, the client can indicate the desired target service(s) where it intends to use that token by way of the resource parameter and can indicate the desired scope of the requested token using the scope parameter. The semantics of such a request are that the client is asking for a token with the requested scope that is usable at all the requested target services. Effectively, the requested access rights of the token are the cartesian product of all the scopes at all the target services. To the extent possible, when issuing access tokens, the authorization server should adapt the scope value associated with an access token to the value the respective resource is able to process and needs to know. This further improves privacy as scope values give an indication of what services the resource owner uses and it improves security as scope values may contain confidential data. As specified in Section 5.1 of [RFC6749], the authorization server must indicate the access token's effective scope to the client in the scope response parameter value when it differs from the scope requested by the client.

  POST /as/token.oauth2 HTTP/1.1
  Host: authorization-server.example.com
  Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
  Content-Type: application/x-www-form-urlencoded


Figure 3: Access Token Request

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-cache, no-store


Figure 4: Access Token Response

Following from the code flow authorization request shown in Figure 2, the below examples show an authorization_code grant type access token request and response where the client tells the authorization server that it wants the access token for use at https://cal.example.com/ (extra line breaks and indentation are for display purposes only).

  POST /as/token.oauth2 HTTP/1.1
  Host: authorization-server.example.com
  Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
  Content-Type: application/x-www-form-urlencoded


Figure 5: Access Token Request

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-cache, no-store


Figure 6: Access Token Response

A subsequent access token request, using the refresh token, where the client tells the authorization server that it wants an access token for use at https://contacts.example.com/ is shown in Figure 5 below with the response shown in Figure 6 (extra line breaks and indentation are for display purposes only).

3. Security Considerations

An access token that is audience restricted to a protected resource that obtains that token legitimately cannot be used to access resources on behalf of the resource owner at other protected resources. The resource parameter enables a client to indicate the protected resources where the requested access token will be used, which in turn enables the authorization server to apply the appropriate audience restrictions to the token.

Some servers may host user content or be multi-tenant. In order to avoid attacks that might confuse a client into sending an access token to a resource that is user controlled or is owned by a different tenant, it is important to use a specific resource URI including a path component. This will cause any access token issued for accessing the user controlled resource to have a invalid audience if replayed against the legitimate resource API.

Although multiple occurrences of the resource parameter may be included in a request, using only a single resource parameter is encouraged. A bearer token that has multiple intended recipients (audiences) can be used by any one of those recipients at any other. Thus, a high degree of trust between the involved parties is needed when using access tokens with multiple audiences. Furthermore an authorization server may be unwilling or unable to fulfill a token request with multiple resources.

4. IANA Considerations

4.1. OAuth Parameters Registration

This specification registers the following value in the IANA "OAuth Parameters" registry [IANA.OAuth.Parameters] established by [RFC6749].

4.2. OAuth Extensions Error Registration

This specification registers the following error in the IANA "OAuth Extensions Error Registry" [IANA.OAuth.Parameters] established by [RFC6749].

5. References

5.1. Normative References

[IANA.OAuth.Parameters] IANA, "OAuth Parameters"
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

5.2. Informative References

[I-D.ietf-oauth-jwsreq] Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization Framework: JWT Secured Authorization Request (JAR)", Internet-Draft draft-ietf-oauth-jwsreq-16, April 2018.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012.
[RFC7519] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015.
[RFC7644] Hunt, P., Grizzle, K., Ansari, M., Wahlstroem, E. and C. Mortimore, "System for Cross-domain Identity Management: Protocol", RFC 7644, DOI 10.17487/RFC7644, September 2015.
[RFC7662] Richer, J., "OAuth 2.0 Token Introspection", RFC 7662, DOI 10.17487/RFC7662, October 2015.

Appendix A. Acknowledgements

This specification was developed within the OAuth Working Group under the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with Eric Rescorla and Benjamin Kaduk serving as Security Area Directors. Additionally, the following individuals contributed ideas, feedback, and wording that helped shape this specification:

Sergey Beryozkin, William Denniss, Vladimir Dzhuvinov, George Fletcher, Dick Hardt, Phil Hunt, Michael Jones, Torsten Lodderstedt, Anthony Nadalin, Justin Richer, Nat Sakimura, Filip Skokan, and Hans Zandbelt.

Appendix B. Document History

[[ to be removed by the RFC Editor before publication as an RFC ]]






Authors' Addresses

Brian Campbell Ping Identity EMail: brian.d.campbell@gmail.com
John Bradley Yubico EMail: ve7jtb@ve7jtb.com
Hannes Tschofenig Arm Limited EMail: hannes.tschofenig@gmx.net