Updates to Audience Values for OAuth 2.0 Authorization Servers

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Table of Contents

1. Introduction

Multiple OAuth 2.0 specifications use tokens (also known as "assertions") that are sent to authorization servers. These tokens contain an audience value or values intended to identify the recipients that the token is intended for. When the token is a JSON Web Token (JWT) [JWT], the audience value(s) are contained in the aud (audience) claim.¶

When performing a security analysis of a pre-final version of the OpenID Federation specification [OpenID.Federation.ID4], University of Stuttgart security researchers Pedram Hosseyni, Dr. Ralf Küsters, and Tim Würtele discovered a vulnerability affecting multiple OpenID and OAuth specifications caused by ambiguities in the audience values of tokens sent to authorization servers. The vulnerability was disclosed to the OAuth working group in an interim meeting in January 2025 called for that purpose, including providing a description of the vulnerability [private_key_jwt.Disclosure]. A paper they published describing the attack is [Audience.Injection].¶

This specification updates the affected OAuth specifications to address the security vulnerability identified. Specifically, it eliminates former ambiguities in the audience values of tokens sent to OAuth 2.0 authorization servers.¶

A general description of the update made to each specification is for it to require that the issuer identifier URL of the authorization server, as defined in [RFC8414], be used as the sole value of the token audience. Furthermore, the authorization server MUST reject any such token that does not contain its own issuer identifier as the sole audience value. An explicit type for each affected kind of token, as defined in [RFC8725], is also defined to facilitate distinguishing between tokens produced in accordance with specifications published prior to these updates and those incorporating them. Specific updates made to each affected specification follow.¶

2. Updates to RFC 7521

This section updates "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7521] to tighten its audience requirements.¶

The description of the Audience parameter in Section 5.1 of [RFC7521] (Assertion Metamodel) is replaced by:¶

Audience

A value that identifies the party intended to process the assertion. The audience MUST contain the issuer identifier [RFC8414] of the authorization server as its sole value. Unlike the audience value specified in [RFC7521], there MUST be no value other than the issuer identifier of the intended authorization server used as the audience of the assertion; this includes that the token endpoint URL of the authorization server MUST NOT be used as an audience value.¶

The description of the Audience parameter in Section 5.2 of [RFC7521] (General Assertion Format and Processing Rules) is replaced by:¶

• The assertion MUST contain an audience that identifies the authorization server as the intended audience, with the issuer identifier [RFC8414] of the authorization server as its sole value. The authorization server MUST reject any assertion that does not contain its own issuer identifier as the sole audience value.¶

In the list of agreements required by participants in Section 7 of [RFC7521] (Interoperability Considerations), "audience identifiers" is removed from the list.¶

3. Updates to RFC 7522

This section updates "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7522] to tighten its audience requirements.¶

The description of the Audience element in Item 2 of Section 3 of [RFC7522] (Assertion Format and Processing Requirements) is replaced by:¶

• The Assertion MUST contain a <Conditions> element with an <AudienceRestriction> element with a single <Audience> element that identifies the authorization server as the intended audience. The value of the <Audience> element MUST be the issuer identifier [RFC8414] of the authorization server. Section 2.5.1.4 of "Assertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0" [OASIS.saml-core-2.0-os] defines the <AudienceRestriction> and <Audience> elements. Unlike the audience value specified in [RFC7522], there MUST be no value other than the issuer identifier of the intended authorization server used as the audience of the assertion; this includes that the token endpoint URL of the authorization server MUST NOT be used as an audience value.¶

  The authorization server MUST reject any assertion that does not contain its own issuer identifier as the sole audience value.¶

In Section 4 of [RFC7522] (Authorization Grant Example), the sentence:¶

• The intended audience of the Assertion is https://saml-sp.example.net, which is an identifier for a SAML Service Provider with which the authorization server identifies itself.¶

is replaced by:¶

• The intended audience of the Assertion is https://authz.example.net, which is the authorization server's issuer identifier.¶

In the same section, the SAML 2.0 Assertion example is replaced by:¶

  <Assertion IssueInstant="2024-11-17T00:53:34.619Z"
    ID="ef1xsbZxPV2oqjd7HTLRLIBlBb7"
    Version="2.0"
    xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
   <Issuer>https://saml-idp.example.com</Issuer>
   <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    [...omitted for brevity...]
   </ds:Signature>
   <Subject>
    <NameID
     Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
     brian@example.com
    </NameID>
    <SubjectConfirmation
      Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
     <SubjectConfirmationData
       NotOnOrAfter="2024-11-17T00:58:34.619Z"
       Recipient="https://authz.example.net/token.oauth2"/>
     </SubjectConfirmation>
    </Subject>
    <Conditions>
      <AudienceRestriction>
        <Audience>https://authz.example.net</Audience>
      </AudienceRestriction>
    </Conditions>
    <AuthnStatement AuthnInstant="2024-11-17T00:53:34.371Z">
      <AuthnContext>
        <AuthnContextClassRef>
          urn:oasis:names:tc:SAML:2.0:ac:classes:X509
        </AuthnContextClassRef>
      </AuthnContext>
    </AuthnStatement>
  </Assertion>

In the list of agreements required by participants in Section 5 of [RFC7521] (Interoperability Considerations), "audience identifiers" is removed from the list.¶

4. Updates to RFC 7523

This section updates "JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7523] to tighten its audience requirements.¶

In Section 3 of [RFC7523] (JWT Format and Processing Requirements), Item 3, which describes the audience value, is replaced by:¶

• The JWT MUST contain an aud (audience) claim containing the issuer identifier [RFC8414] of the authorization server as its sole value. The authorization server MUST have an issuer identifier to be used with this specification. Unlike the aud value specified in [RFC7523], there MUST be no value other than the issuer identifier of the intended authorization server used as the audience of the JWT; this includes that the token endpoint URL of the authorization server MUST NOT be used as an audience value. To simplify implementations, the aud claim value MUST be a JSON string, and not a single-valued JSON array. The authorization server MUST reject any JWT that does not contain its issuer identifier as its sole audience value. In the absence of an application profile specifying otherwise, compliant applications MUST compare the audience values using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986].¶

In Section 3.1 of [RFC7523] (Authorization Grant Processing), the following requirement is added:¶

• Authorization grant JWTs MUST be explicitly typed by using the typ header parameter value authorization-grant+jwt or another more specific explicit type value defined by a specification profiling this specification. Authorization grant JWTs not using the explicit type value MUST be rejected by the authorization server.¶

In Section 3.2 of [RFC7523] (Client Authentication Processing), the following requirement is added:¶

• Client authentication JWTs MUST be explicitly typed by using the typ header parameter value client-authentication+jwt another more specific explicit type value defined by a specification profiling this specification. Client authentication JWTs not using the explicit type value MUST be rejected by the authorization server.¶

In Section 4 of [RFC7523] (Authorization Grant Example), the sentence:¶

• The intended audience of the JWT is https://jwt-rp.example.net, which is an identifier with which the authorization server identifies itself.¶

is replaced by:¶

• The intended audience of the JWT is https://authz.example.net, which is the authorization server's issuer identifier.¶

In the same section, the JWT Claims Set example is replaced by:¶

  {"aud":"https://authz.example.net",
   "iss":"https://jwt-idp.example.com",
   "sub":"mailto:mike@example.com",
   "iat":1731721541,
   "exp":1731725141,
   "http://claims.example.com/member":true
  }

In the same section, the sentence:¶

• The following example JSON object, used as the header of a JWT, declares that the JWT is signed with the Elliptic Curve Digital Signature Algorithm (ECDSA) P-256 SHA-256 using a key identified by the kid value 16.¶

is replaced by:¶

• The following example JSON object, used as the header parameters of a JWT, declares that the JWT is an authorization grant JWT, is signed with the Elliptic Curve Digital Signature Algorithm (ECDSA) P-256 with SHA-256, and was signed with a key identified by the kid value 16.¶

In the same section, the JOSE Header Parameters example is replaced by:¶

  {"typ":"authorization-grant+jwt","alg":"ES256","kid":"16"}

In the same section, the example POST body is replaced by:¶

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer
  &assertion=eyJ0eXAiOiJhdXRob3JpemF0aW9uLWdyYW50K2p3dCIsImFsZyI6Ik
    VTMjU2Iiwia2lkIjoiMTYifQ.
  eyJhdWQiOiJodHRwczovLw[...omitted for brevity...].
  J9l-ZhwP[...omitted for brevity...]

In the list of agreements required by participants in Section 5 of [RFC7523] (Interoperability Considerations), "audience identifiers" is removed from the list.¶

5. Updates to RFC 9126

This section updates "OAuth 2.0 Pushed Authorization Requests" [RFC9126] to tighten its audience requirements.¶

The paragraph describing the audience value in Section 2 of [RFC9126] (Pushed Authorization Request Endpoint) is replaced by:¶

• This update resolves the potential ambiguity regarding the appropriate audience value to use when employing JWT client assertion-based authentication (as defined in Section 2.2 of [RFC7523] with the private_key_jwt or client_secret_jwt authentication method names per Section 9 of [OpenID.Core]) that was described in [RFC9126]. To address that ambiguity, the issuer identifier URL of the authorization server according to [RFC8414] MUST be used as the sole value of the audience. The authorization server MUST reject any such JWT that does not contain its own issuer identifier as the sole audience value.¶

6. Security Considerations

The security considerations described within the following specifications are all applicable to this document: "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7521], "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7522], "JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7523], "OAuth 2.0 Pushed Authorization Requests" [RFC9126], "The OAuth 2.0 Authorization Framework" [RFC6749], and "JSON Web Token (JWT)" [JWT].¶

This specification tightens token audience requirements to prevent attacks that could result from exploiting audience ambiguities previously allowed by [RFC7521], [RFC7522], [RFC7523], and [RFC9126]. These attacks are described in [private_key_jwt.Disclosure] and [Audience.Injection].¶

7. IANA Considerations

Appendix A. Document History

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

-01¶

• Reworked to make updates to RFC 7523, rather than replacing it.¶

• Removed updates to RFC 9101.¶

• Added reference to the University of Stuttgart paper [Audience.Injection].¶

-00¶

• Initial working group draft, replacing draft-jones-oauth-rfc7523bis-00.¶

Acknowledgements

We would like to acknowledge the contributions of the following people to this specification: John Bradley, Ralph Bragg, Joseph Heenan, Pedram Hosseyni, Aaron Parecki, Filip Skokan, and Tim Würtele.¶

Authors' Addresses