| RFC 9701 | JWT Response | December 2024 |
| Lodderstedt & Dzhuvinov | Standards Track | [Page] |
This specification proposes an additional response secured by JSON Web Token (JWT) for OAuth 2.0 Token Introspection.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9701.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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"OAuth 2.0 Token Introspection" [RFC7662] specifies a method for a protected resource to query an OAuth 2.0 authorization server to determine the state of an access token and obtain data associated with the access token. This enables deployments to implement opaque access tokens in an interoperable way.¶
The introspection response, as specified in "OAuth 2.0 Token Introspection" [RFC7662], is a plain JSON object. However, there are use cases where the resource server requires stronger assurance that the authorization server issued the token introspection response for an access token, including cases where the authorization server assumes liability for the content of the token introspection response. An example is a resource server using verified personal data to create certificates, which in turn are used to create qualified electronic signatures.¶
In such use cases, it may be useful or even required to return a signed JWT [RFC7519] as the introspection response. This specification extends the token introspection endpoint with the capability to return responses as JWTs.¶
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.¶
The authorization server (AS) and the resource server (RS) maintain a strong, two-way trust relationship. The resource server relies on the authorization server to obtain authorization, user, and other data as input to its access control decisions and service delivery. The authorization server relies on the resource server to handle the provided data appropriately.¶
In the context of this specification, the token introspection endpoint is used to convey such security data and potentially also privacy-sensitive data related to an access token.¶
In order to process the introspection requests in a secure and privacy-preserving manner, the authorization server MUST be able to identify, authenticate, and authorize resource servers.¶
The AS MAY additionally encrypt the token introspection response JWTs. If encryption is used, the AS is provisioned with encryption keys and algorithms for the RS.¶
The AS MUST be able to determine whether an RS is the audience for a particular access token and what data it is entitled to receive; otherwise, the RS is not authorized to obtain data for the access token. The AS has the discretion of how to fulfill this requirement. The AS could, for example, maintain a mapping between scope values and RSs.¶
The requirements given above imply that the AS maintains credentials and other configuration data for each RS.¶
One way is by utilizing dynamic client registration [RFC7591] and treating every RS as an OAuth client. In this case, the AS is assumed to at least maintain a "client_id" and a "token_endpoint_auth_method" with complementary authentication method metadata, such as "jwks" or "client_secret". In cases where the AS needs to acquire consent to transmit data to an RS, the following client metadata fields are recommended: "client_name", "client_uri", "contacts", "tos_uri", and "policy_uri".¶
The AS MUST restrict the use of client credentials by an RS to the calls it requires, e.g., the AS MAY restrict such a client to call the token introspection endpoint only. How the AS implements this restriction is beyond the scope of this specification.¶
This specification further introduces client metadata to manage the configuration options required to sign and encrypt token introspection response JWTs.¶
An RS requests a JWT introspection response by sending an introspection request
with an Accept HTTP header field set to
"application/token-introspection+jwt".¶
The AS MUST authenticate the caller at the token introspection endpoint. Authentication can utilize client authentication methods or a separate access token that is issued to the RS and identifies the RS as the subject.¶
The following is a non-normative example request, with the RS authenticating with a private key JWT:¶
The introspection endpoint responds with a JWT, setting the
Content-Type HTTP header field to
"application/token-introspection+jwt" and the JWT typ
("type") header parameter to "token-introspection+jwt".¶
The JWT MUST include the following top-level claims:¶
A JSON object containing the members of the token introspection response, as specified in [RFC7662], Section 2.2. The separation of the introspection response members into a dedicated JSON object containing a JWT claim is intended to prevent conflict and confusion with top-level JWT claims that may bear the same name.¶
If the access token is invalid, expired, revoked, or not intended for the
calling resource server (audience), the authorization server MUST set the value of the
active member in the token_introspection
claim to false and MUST NOT include other members.
Otherwise, the active member is set to true.¶
The AS SHOULD narrow down the scope value to the scopes
relevant to the particular RS.¶
As specified in Section 2.2 of [RFC7662], implementations MAY extend the
token introspection response with service-specific claims. In the context of this
specification, such claims will be added as top-level members of the
token_introspection claim.¶
Token introspection response parameter names intended to be used across domains MUST be registered in the "OAuth Token Introspection Response" registry [IANA.OAuth.Token.Introspection] defined by [RFC7662].¶
When the AS acts as a provider of resource owner identity claims to the RS, the AS determines, based on its RS-specific policy, what identity claims to return in the token introspection response. The AS MUST ensure the release of any privacy-sensitive data is legally based (see Section 9).¶
Further content of the introspection response is determined by the RS-specific policy at the AS.¶
The JWT MAY include other claims, including those from the
"JSON Web Token Claims" registry established by [RFC7519]. The JWT SHOULD NOT include the
sub and exp claims, as an additional measure to prevent misuse of the JWT as an access token (see
Section 8.1).¶
Note: Although the JWT format is widely used as an access token format, the JWT returned in the introspection response is not an alternative representation of the introspected access token and is not intended to be used as an access token.¶
This specification registers the "application/token-introspection+jwt" media type,
which is used as the value of the typ ("type") header
parameter of the JWT to indicate that the payload is a token introspection response.¶
The JWT is cryptographically secured as specified in [RFC7519].¶
Depending on the specific resource server policy, the JWT is either signed or signed and encrypted. If the JWT is signed and encrypted, it MUST be a Nested JWT, as defined in JWT [RFC7519].¶
Note: An AS compliant with this specification MUST refuse to serve introspection requests that don't authenticate the caller and return an HTTP status code 400. This is done to ensure token data is released to legitimate recipients only and prevent downgrading to [RFC7662] behavior (see Section 8.2).¶
The following is a non-normative example response (with line breaks for display purposes only):¶
The example response JWT header contains the following JSON document:¶
{
"typ": "token-introspection+jwt",
"alg": "RS256",
"kid": "wG6D"
}
¶
The example response JWT payload contains the following JSON document:¶
{
"iss":"https://as.example.com/",
"aud":"https://rs.example.com/resource",
"iat":1514797892,
"token_introspection":
{
"active":true,
"iss":"https://as.example.com/",
"aud":"https://rs.example.com/resource",
"iat":1514797822,
"exp":1514797942,
"client_id":"paiB2goo0a",
"scope":"read write dolphin",
"sub":"Z5O3upPC88QrAjx00dis",
"birthdate":"1982-02-01",
"given_name":"John",
"family_name":"Doe",
"jti":"t1FoCCaZd4Xv4ORJUWVUeTZfsKhW30CQCrWDDjwXy6w"
}
}
¶
The authorization server determines the algorithm to secure the JWT for a particular introspection response. This decision can be based on registered metadata parameters for the resource server, supplied via dynamic client registration [RFC7591] with the resource server acting as a client, as specified below.¶
The parameter names follow the pattern established by OpenID Connect Dynamic Client Registration [OpenID.Registration] for configuring signing and encryption algorithms for JWT responses at the UserInfo endpoint.¶
The following client metadata parameters are introduced by this specification:¶
alg value), as defined in
"JSON Web Algorithms (JWA)" [RFC7518], for signing
introspection responses. If
this is specified, the response will be signed using JWS and the
configured algorithm. The default, if omitted, is RS256.¶
alg value),
as defined in JWA [RFC7518], for
content key encryption.
If this is specified, the response will be encrypted using JWE and the
configured content encryption algorithm
(introspection_encrypted_response_enc). The default,
if omitted, is that no encryption is performed.
If both signing and
encryption are requested, the response will be
signed then encrypted, with the result being a Nested JWT, as defined in
JWT [RFC7519].¶
enc value),
as defined in JWA [RFC7518], for
content encryption of
introspection responses. The default, if omitted, is A128CBC-HS256.
Note: This parameter MUST NOT be specified without setting
introspection_encrypted_response_alg.¶
Resource servers may register their public encryption keys
using the jwks_uri or jwks
metadata parameters.¶
Authorization servers SHOULD publish the supported algorithms for signing and encrypting the JWT of an introspection response by utilizing "OAuth 2.0 Authorization Server Metadata" [RFC8414] parameters. Resource servers use this data to parametrize their client registration requests.¶
The following parameters are introduced by this specification:¶
alg values), as defined in
JWA [RFC7518], supported by the introspection
endpoint to sign the response.¶
alg values), as defined in
JWA [RFC7518], supported by the
introspection endpoint to encrypt the content encryption key for
introspection responses (content key encryption).¶
enc values), as defined in
JWA [RFC7518], supported by the introspection
endpoint to encrypt the response (content encryption).¶
The iss and potentially the aud
claim of a token introspection JWT can resemble those of a JWT-encoded access token.
An attacker could try to exploit this and pass a JWT token introspection response as
an access token to the resource server. The typ ("type")
JWT header "token-introspection+jwt" and the encapsulation of the token introspection members,
such as sub and scope in
the token_introspection claim, are intended to prevent such
substitution attacks. Resource servers MUST therefore check the typ
JWT header value of received JWT-encoded access tokens and ensure all minimally
required claims for a valid access token are present.¶
Resource servers MUST additionally apply the countermeasures against access token replay, as described in [RFC9700].¶
JWT confusion and other attacks involving JWTs are discussed in [RFC8725].¶
The authorization server MUST use Transport Layer Security (TLS) 1.2 (or higher), per BCP 195 [RFC9325], in order to prevent token data leakage.¶
Section 2.1 of [RFC7662] permits requests to the introspection endpoint to be authorized with an access token that doesn't identify the caller. To prevent introspection of tokens by parties that are not the intended consumer, the authorization server MUST require all requests to the token introspection endpoint to be authenticated.¶
The token introspection response can be used to transfer personal identifiable information (PII) from the AS to the RS. The AS MUST conform to legal and jurisdictional constraints for the data transfer before any data is released to a particular RS. The details and determining of these constraints vary by jurisdiction and are outside the scope of this document.¶
A commonly found way to establish the legal basis for releasing PII is by explicit user consent gathered from the resource owner by the AS during the authorization flow.¶
It is also possible that the legal basis is established out of band, for example, in an explicit contract or by the client gathering the resource owner's consent.¶
If the AS and the RS belong to the same legal entity (1st party scenario), there is potentially no need for an explicit user consent, but the terms of service and policy of the respective service provider MUST be enforced at all times.¶
In any case, the AS MUST ensure that the scope of the legal basis is enforced throughout the whole process. The AS MUST retain the scope of the legal basis with the access token, e.g., in the scope value, it MUST authenticate the RS, and the AS MUST determine the data an RS is allowed to receive based on the RS's identity and suitable token data, e.g., the scope value.¶
Implementers should be aware that a token introspection request lets the AS know when the client (and potentially the user) is accessing the RS, which is also an indication of when the user is using the client. If this implication is not acceptable, implementers MUST use other means to relay access token data, for example, by directly transferring the data needed by the RS within the access token.¶
The following client metadata definitions have been registered in the IANA "OAuth Dynamic Client Registration Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591]:¶
introspection_signed_response_alg¶
The following values have been registered in the IANA "OAuth Authorization Server Metadata" registry [IANA.OAuth.Parameters] established by [RFC8414].¶
introspection_signing_alg_values_supported¶
The "application/token-introspection+jwt" media type has been registered in the "Media Types" registry [IANA.MediaTypes] in the manner described in [RFC6838]. It can be used to indicate that the content is a token introspection response in JWT format.¶
The "token_introspection" claim has been registered in the "JSON Web Token (JWT)" registry [IANA.JWT] in the manner described in [RFC7519].¶
We would like to thank Petteri Stenius, Neil Madden, Filip Skokan, Tony Nadalin, Remco Schaar, Justin Richer, Takahiko Kawasaki, Benjamin Kaduk, Robert Wilton, and Roman Danyliw for their valuable feedback.¶