JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants

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Table of Contents

1. Introduction

JSON Web Token (JWT) [JWT] is a JSON-based [RFC7159] security token encoding that enables identity and security information to be shared across security domains. A security token is generally issued by an Identity Provider and consumed by a Relying Party that relies on its content to identify the token's subject for security-related purposes.¶

The OAuth 2.0 Authorization Framework [RFC6749] provides a method for making authenticated HTTP requests to a resource using an access token. Access tokens are issued to third-party clients by an authorization server (AS) with the (sometimes implicit) approval of the resource owner. In OAuth, an authorization grant is an abstract term used to describe intermediate credentials that represent the resource owner authorization. An authorization grant is used by the client to obtain an access token. Several authorization grant types are defined to support a wide range of client types and user experiences. OAuth also allows for the definition of new extension grant types to support additional clients or to provide a bridge between OAuth and other trust frameworks. Finally, OAuth allows the definition of additional authentication mechanisms to be used by clients when interacting with the authorization server.¶

"Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7521] is an abstract extension to OAuth 2.0 that provides a general framework for the use of assertions (a.k.a. security tokens) as client credentials and/or authorization grants with OAuth 2.0. This specification profiles the OAuth Assertion Framework [RFC7521] to define an extension grant type that uses a JWT Bearer Token to request an OAuth 2.0 access token as well as for use as client credentials. The format and processing rules for the JWT defined in this specification are intentionally similar, though not identical, to those in the closely related specification "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7522]. The differences arise where the structure and semantics of JWTs differ from SAML Assertions. JWTs, for example, have no direct equivalent to the <SubjectConfirmation> or <AuthnStatement> elements of SAML Assertions.¶

This document defines how a JWT Bearer Token can be used to request an access token when a client wishes to utilize an existing trust relationship, expressed through the semantics of the JWT, without a direct user-approval step at the authorization server. It also defines how a JWT can be used as a client authentication mechanism. The use of a security token for client authentication is orthogonal to and separable from using a security token as an authorization grant. They can be used either in combination or separately. Client authentication using a JWT is nothing more than an alternative way for a client to authenticate to the token endpoint or other endpoints such as the pushed authorization endpoint [RFC9126] and must be used in conjunction with some grant type to form a complete and meaningful protocol request. JWT authorization grants may be used with or without client authentication or identification. Whether or not client authentication is needed in conjunction with a JWT authorization grant, as well as the supported types of client authentication, are policy decisions at the discretion of the authorization server.¶

The process by which the client obtains the JWT, prior to exchanging it with the authorization server or using it for client authentication, is out of scope.¶

2. HTTP Parameter Bindings for Transporting Assertions

The OAuth Assertion Framework [RFC7521] defines generic HTTP parameters for transporting assertions (a.k.a. security tokens) during interactions with a token endpoint. This section defines specific parameters and treatments of those parameters for use with JWT Bearer Tokens.¶

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

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer
  &assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.
  eyJpc3Mi[...omitted for brevity...].
  J9l-ZhwP[...omitted for brevity...]

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

  grant_type=authorization_code&
  code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3A
  client-assertion-type%3Ajwt-bearer&
  client_assertion=eyJhbGciOiJSUzI1NiIsImtpZCI6IjIyIn0.
  eyJpc3Mi[...omitted for brevity...].
  cC4hiUPo[...omitted for brevity...]

3. JWT Format and Processing Requirements

In order to issue an access token response as described in OAuth 2.0 [RFC6749] or to rely on a JWT for client authentication, the authorization server MUST validate the JWT according to the criteria below. Application of additional restrictions and policy are at the discretion of the authorization server.¶

1. The JWT MUST contain an iss (issuer) claim that contains a unique identifier for the entity that issued the JWT. In the absence of an application profile specifying otherwise, compliant applications MUST compare issuer values using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986].¶

2. The JWT MUST contain a sub (subject) claim identifying the principal that is the subject of the JWT. Two cases need to be differentiated:¶

   1. For the authorization grant, the subject typically identifies an authorized accessor for which the access token is being requested (i.e., the resource owner or an authorized delegate), but in some cases, may be a pseudonymous identifier or other value denoting an anonymous user.¶

   2. For client authentication, the subject MUST be the client_id of the OAuth client.¶

3. 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].¶

4. The JWT MUST contain an exp (expiration time) claim that limits the time window during which the JWT can be used. The authorization server MUST reject any JWT with an expiration time that has passed, subject to allowable clock skew between systems. Note that the authorization server may reject JWTs with an exp claim value that is unreasonably far in the future.¶

5. The JWT MAY contain an nbf (not before) claim that identifies the time before which the token MUST NOT be accepted for processing.¶

6. The JWT MAY contain an iat (issued at) claim that identifies the time at which the JWT was issued. Note that the authorization server may reject JWTs with an iat claim value that is unreasonably far in the past.¶

7. The JWT MAY contain a jti (JWT ID) claim that provides a unique identifier for the token. The authorization server MAY ensure that JWTs are not replayed by maintaining the set of used jti values for the length of time for which the JWT would be considered valid based on the applicable exp instant.¶

8. The JWT MAY contain other claims.¶

9. The JWT MUST be digitally signed or have a Message Authentication Code (MAC) applied by the issuer. The authorization server MUST reject JWTs with an invalid signature or MAC.¶

10. The authorization server MUST reject a JWT that is not valid in all other respects per "JSON Web Token (JWT)" [JWT].¶

  HTTP/1.1 400 Bad Request
  Content-Type: application/json
  Cache-Control: no-store

  {
   "error":"invalid_grant",
   "error_description":"Audience validation failed"
  }

4. Authorization Grant Example

The following examples illustrate what a conforming JWT and an access token request would look like.¶

The example shows a JWT issued and signed by the system entity identified as https://jwt-idp.example.com. The subject of the JWT is identified by email address as mike@example.com. The intended audience of the JWT is https://authz.example.net, which is the authorization server's issuer identifier. The JWT is sent as part of an access token request to the authorization server's token endpoint at https://authz.example.net/token.oauth2.¶

Below is an example JSON object that could be encoded to produce the JWT Claims Set for a JWT:¶

  {"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
  }

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.¶

  {"alg":"ES256","kid":"16"}

To present the JWT with the claims and header shown in the previous example as part of an access token request, for example, the client might make the following HTTPS request (with extra line breaks for display purposes only):¶

  POST /token.oauth2 HTTP/1.1
  Host: authz.example.net
  Content-Type: application/x-www-form-urlencoded

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer
  &assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.
  eyJpc3Mi[...omitted for brevity...].
  J9l-ZhwP[...omitted for brevity...]

5. Interoperability Considerations

Agreement between system entities regarding identifiers, keys, and endpoints is required in order to achieve interoperable deployments of this profile. Specific items that require agreement include values for the issuer identifiers, the locations of endpoints, the key used to apply and verify the digital signature or MAC over the JWT, one-time use restrictions on the JWT, maximum JWT lifetime allowed, and the specific subject and claim requirements of the JWT. The exchange of such information is explicitly out of scope for this specification. In some cases, additional profiles may be created that constrain or prescribe these values or specify how they are to be exchanged. Examples of such profiles include the OAuth 2.0 Dynamic Client Registration Protocol [RFC7591], OAuth 2.0 Authorization Server Metadata [RFC8414], OpenID Connect Dynamic Client Registration 1.0 [OpenID.Registration], OpenID Connect Discovery 1.0 [OpenID.Discovery], and OpenID Federation 1.0 [OpenID.Federation].¶

The RS256 algorithm, from [JWA], is a mandatory-to-implement JSON Web Signature algorithm for this profile.¶

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], "The OAuth 2.0 Authorization Framework" [RFC6749], and "JSON Web Token (JWT)" [JWT].¶

The specification does not mandate replay protection for the JWT usage for either the authorization grant or for client authentication. It is an optional feature, which implementations may employ at their own discretion.¶

This specification tightens the JWT audience requirements to prevent attacks that could result from exploiting audience ambiguities allowed by [RFC7523].¶

7. Privacy Considerations

A JWT may contain privacy-sensitive information and, to prevent disclosure of such information to unintended parties, should only be transmitted over encrypted channels, such as Transport Layer Security (TLS). In cases where it is desirable to prevent disclosure of certain information to the client, the JWT should be encrypted to the authorization server.¶

Deployments should determine the minimum amount of information necessary to complete the exchange and include only such claims in the JWT. In some cases, the sub (subject) claim can be a value representing an anonymous or pseudonymous user, as described in Section 6.3.1 of the OAuth Assertion Framework [RFC7521].¶

8. IANA Considerations

The IANA actions of registering the URNs urn:ietf:params:oauth:grant-type:jwt-bearer and urn:ietf:params:oauth:client-assertion-type:jwt-bearer in the IANA "OAuth URI" registry [IANA.OAuth.Parameters] established by "An IETF URN Sub-Namespace for OAuth" [RFC6755] were performed by [RFC7523]. No IANA actions are requested by this specification.¶

Appendix A. Document History

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

-00¶

• Initial draft starting with rfc7523.xml, updated to use current references and affiliations and modern xml2rfc syntax, removing the IANA actions already performed, and adding the Document History section.¶

• Use AS issuer identifier as the sole audience value.¶

Acknowledgements

This profile was derived from "Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7522], which has the same authors as this document.¶

Authors' Addresses