| OAuth JARM | December 2024 | |
| Lodderstedt & Campbell | Standards Track | [Page] |
This document defines a new JWT-based mode to encode OAuth authorization responses. Clients are enabled to request the transmission of the authorization response parameters along with additional data in JWT format. This mechanism enhances the security of the standard authorization response with support for signing and optional encryption of the response. A signed response provides message integrity, sender authentication, audience restriction, and protection from mix-up attacks. Encrypting the response provides confidentiality of the response parameter values. The JWT authorization response mode can be used in conjunction with any response type.¶
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.¶
For the purpose of this document, the terms defined in [RFC6749] and [OIDC] apply.¶
This document defines a new JWT-based [RFC7519] mode to encode OAuth [RFC6749] authorization response parameters. All response parameters defined for a given response type are conveyed in a JWT along with additional claims used to further protect the transmission. Since there are different techniques to encode the JWT itself in the response to the client, namely query URI parameter, fragment component and form post, this document defines a set of response mode values in accordance with [OIDM] corresponding to these techniques.¶
The JWT always contains the following data utilized to secure the transmission:¶
iss - the issuer URL of the authorization server that created the response¶
aud - the client_id of the client the response is intended for¶
exp - expiration of the JWT. A maximum JWT lifetime of 10 minutes is RECOMMENDED.¶
The JWT MUST furthermore contain the authorization endpoint response parameters as defined for the particular response types, even in case of an error response. Authorization endpoint response parameter names and string values are included as JSON strings and numerical values (e.g., expires_in value) are included as JSON numbers. This pattern is applicable to all response types including those defined in [OIDM]. The following subsection illustrates the pattern for the response type "code".¶
Note: Additional authorization endpoint response parameters defined by extensions, e.g. session_state as defined in [OISM], will also be added to the JWT.¶
The JWT response document MAY contain further element, e.g. the claims defined in [RFC7519]. Implementation SHOULD adhere to the respective processing rules and ignore unrecognized elements.¶
For the grant type authorization "code" the JWT contains the response parameters as defined in [RFC6749], sections 4.1.2:¶
code - the authorization code¶
state - the state value as sent by the client in the authorization request, if the client included a state parameter¶
The following example shows the JWT claims for a successful "code" authorization response:¶
{
"iss":"https://accounts.example.com",
"aud":"s6BhdRkqt3",
"exp":1311281970,
"code":"PyyFaux2o7Q0YfXBU32jhw.5FXSQpvr8akv9CeRDSd0QA",
"state":"S8NJ7uqk5fY4EjNvP_G_FtyJu6pUsvH9jsYni9dMAJw"
}
¶
In case of an error response, the JWT contains the error response parameters as defined in [RFC6749], sections 4.1.2.1:¶
error - the error code¶
error_description (OPTIONAL) - a human readable description of the error¶
error_uri (OPTIONAL) - a URI identifying a human-readable web page with information about the error¶
state - the state value as sent by the client in the authorization request (if applicable)¶
The following example shows the JWT payload for such an error response:¶
{
"iss":"https://accounts.example.com",
"aud":"s6BhdRkqt3",
"exp":1311281970,
"error":"access_denied",
"state":"S8NJ7uqk5fY4EjNvP_G_FtyJu6pUsvH9jsYni9dMAJw"
}
¶
The JWT is either signed, or signed and encrypted. If the JWT is both signed and encrypted, the JSON document will be signed then encrypted, with the result being a Nested JWT, as defined in [RFC7519].¶
The authorization server determines what algorithm to employ to secure the JWT for a particular authorization response. This decision can be based on registered metadata parameters for the client as defined by this document (see Section 3).¶
For guidance on key management in general and especially on use of symmetric algorithms for signing and encrypting based on client secrets see section 10 of [OIDC].¶
This document defines the following response mode values:¶
The response mode "query.jwt" causes the authorization server to send the authorization response as HTTP redirect to the redirect URI of the client. The authorization server adds the parameter response containing the JWT as defined in Section 2.1. to the query component of the redirect URI using the "application/x-www-form-urlencoded" format.¶
This is an example response (line breaks for display purposes only):¶
HTTP/1.1 302 Found Location: https://client.example.com/cb? response=eyJraWQiOiJsYWViIiwiYWxnIjoiRVMyNTYifQ.eyAgImlzcyI6ICJodHRwczovL2FjY291 bnRzLmV4YW1wbGUuY29tIiwgICJhdWQiOiAiczZCaGRSa3F0MyIsICAiZXhwIjogMTMxMTI4MTk3MCwg ICJjb2RlIjogIlB5eUZhdXgybzdRMFlmWEJVMzJqaHcuNUZYU1FwdnI4YWt2OUNlUkRTZDBRQSIsICAi c3RhdGUiOiAiUzhOSjd1cWs1Zlk0RWpOdlBfR19GdHlKdTZwVXN2SDlqc1luaTlkTUFKdyJ9.4VdtknV Z9zFYDVLagJpVBD436bjPMcSgOaPDPFgTEkNyCs2uIHYJ2XML6d2w1AUsm5GBG77DBisZNhLWfug6dA¶
Note: "query.jwt" MUST NOT be used in conjunction with response types that contain "token" or "id_token" unless the response JWT is encrypted to prevent token leakage in the URL.¶
The response mode "fragment.jwt" causes the authorization server to send the authorization response as HTTP redirect to the redirect URI of the client. The authorization server adds the parameter response containing the JWT as defined in Section 2.1. to the fragment component of the redirect URI using the "application/x-www-form-urlencoded" format.¶
This is an example response (line breaks for display purposes only):¶
HTTP/1.1 302 Found Location: https://client.example.com/cb# response=eyJraWQiOiJsYWViIiwiYWxnIjoiRVMyNTYifQ.eyAgImlzcyI6ICJodHRwczovL2FjY291 bnRzLmV4YW1wbGUuY29tIiwgICJhdWQiOiAiczZCaGRSa3F0MyIsICAiZXhwIjogMTMxMTI4MTk3MCwg ICJhY2Nlc3NfdG9rZW4iOiAiMllvdG5GWkZFanIxekNzaWNNV3BBQSIsICAic3RhdGUiOiAiUzhOSjd1 cWs1Zlk0RWpOdlBfR19GdHlKdTZwVXN2SDlqc1luaTlkTUFKdyIsICAidG9rZW5fdHlwZSI6ICJiZWFy ZXIiLCAgImV4cGlyZXNfaW4iOiAzNjAwLCAgInNjb3BlIjogImV4YW1wbGUifQ.g_96IM2t_6Dazm1Jp b2gbO2EXe5IKTw2bYS7l9Y1RI8HbNPYN5EdNjxcWeL5LTQaUAZ2PtJoHbTdjMvNa3xbVg¶
The response mode "form_post.jwt" uses the technique described in [OIFP] to convey the JWT to the client. The response parameter containing the JWT is encoded as HTML form value that is auto-submitted in the User Agent, and thus is transmitted via the HTTP POST method to the Client, with the result parameters being encoded in the body using the "application/x-www-form-urlencoded" format.¶
This is an example response from the authorization server to the user agent (line breaks for display purposes only),¶
HTTP/1.1 200 OK
Content-Type: text/html;charset=UTF-8
Cache-Control: no-cache, no-store
Pragma: no-cache
<html>
<head><title>Submit This Form</title></head>
<body onload="javascript:document.forms[0].submit()">
<form method="post" action="https://client.example.com/cb">
<input type="hidden" name="response"
value="eyJraWQiOiJsYWViIiwiYWxnIjoiRVMyNTYifQ.eyAgImlzcyI6ICJodHRw
czovL2FjY291bnRzLmV4YW1wbGUuY29tIiwgICJhdWQiOiAiczZCaGRSa3F0MyIsIC
AiZXhwIjogMTMxMTI4MTk3MCwgICJhY2Nlc3NfdG9rZW4iOiAiMllvdG5GWkZFanIx
ekNzaWNNV3BBQSIsICAic3RhdGUiOiAiUzhOSjd1cWs1Zlk0RWpOdlBfR19GdHlKdT
ZwVXN2SDlqc1luaTlkTUFKdyIsICAidG9rZW5fdHlwZSI6ICJiZWFyZXIiLCAgImV4
cGlyZXNfaW4iOiAzNjAwLCAgInNjb3BlIjogImV4YW1wbGUifQ.g_96IM2t_6Dazm1
Jpb2gbO2EXe5IKTw2bYS7l9Y1RI8HbNPYN5EdNjxcWeL5LTQaUAZ2PtJoHbTdjMvNa
3xbVg"/>
</form>
</body>
</html>
¶
which results in the following POST request to the client's redirect URI.¶
POST /cb HTTP/1.1 Host: client.example.org Content-Type: application/x-www-form-urlencoded response=eyJraWQiOiJsYWViIiwiYWxnIjoiRVMyNTYifQ.eyAgImlzcyI6ICJodHRw czovL2FjY291bnRzLmV4YW1wbGUuY29tIiwgICJhdWQiOiAiczZCaGRSa3F0MyIsICAi ZXhwIjogMTMxMTI4MTk3MCwgICJhY2Nlc3NfdG9rZW4iOiAiMllvdG5GWkZFanIxekNz aWNNV3BBQSIsICAic3RhdGUiOiAiUzhOSjd1cWs1Zlk0RWpOdlBfR19GdHlKdTZwVXN2 SDlqc1luaTlkTUFKdyIsICAidG9rZW5fdHlwZSI6ICJiZWFyZXIiLCAgImV4cGlyZXNf aW4iOiAzNjAwLCAgInNjb3BlIjogImV4YW1wbGUifQ.g_96IM2t_6Dazm1Jpb2gbO2EX e5IKTw2bYS7l9Y1RI8HbNPYN5EdNjxcWeL5LTQaUAZ2PtJoHbTdjMvNa3xbVg¶
The response mode "jwt" is a shortcut and indicates the default redirect encoding (query, fragment) for the requested response type. The default for response type "code" is "query.jwt" whereas the default for "token" and the response types defined in [OIDM], except "none", is "fragment.jwt".¶
Assumption: the client remembers the authorization server to which it sent the authorization request and binds this information to the user agent.¶
The client MUST process the JWT secured response as follows:¶
kid JWT header parameter. The key might be a private key, where the corresponding public key is registered with the expected issuer of the response ("use":"enc" via the client's metadata jwks or jwks_uri) or a key derived from its client secret (see Section 2.2).¶
iss element from the JWT and checks whether its value is well known and identifies the expected issuer of the authorization process in examination. If the check fails, the client MUST abort processing and refuse the response.¶
aud element from the JWT and checks whether it matches the client id the client used to identify itself in the corresponding authorization request. If the check fails, the client MUST abort processing and refuse the response.¶
exp element to determine if the JWT is still valid. If the check fails, the client MUST abort processing and refuse the response.¶
none ("alg":"none") MUST NOT be accepted. If the check fails, the client MUST abort processing and refuse the response.¶
The client will perform further checks, e.g. for CSRF detection, which are out of scope of this specification. Please see [I-D.ietf-oauth-security-topics] for more security recommendations.¶
Note: The way the client obtains the keys for verifying the JWT's signature (step 5) is out of scope of this document. Established mechanism such as [OIDD] or [RFC8414] SHOULD be utilized.¶
The client MUST NOT process the grant type specific authorization response parameters before all checks succeed.¶
The Dynamic Client Registration Protocol [RFC7591] defines an API for dynamically registering OAuth 2.0 client metadata with authorization servers. The metadata defined by [RFC7591], and registered extensions to it, also imply a general data model for clients that is useful for authorization server implementations even when the Dynamic Client Registration Protocol isn't in play. Such implementations will typically have some sort of user interface available for managing client configuration.¶
The following client metadata parameters are introduced by this specification:¶
authorization_signed_response_algalg algorithm REQUIRED for signing authorization responses. If this is specified, the response will be signed using JWS and the configured algorithm. If unspecified, the default algorithm to use for signing authorization responses is RS256. The algorithm none is not allowed.¶
authorization_encrypted_response_algalg algorithm REQUIRED for encrypting authorization responses. 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]. The default, if omitted, is that no encryption is performed.¶
authorization_encrypted_response_encenc algorithm REQUIRED for encrypting authorization responses. If authorization_encrypted_response_alg is specified, the default for this value is A128CBC-HS256. When authorization_encrypted_response_enc is included, authorization_encrypted_response_alg MUST also be provided.¶
The jwks_uri or jwks metadata parameters can be used by clients to register their public encryption keys.¶
Authorization servers SHOULD publish the supported algorithms for signing and encrypting the JWT of an authorization response by utilizing OAuth 2.0 Authorization Server Metadata [RFC8414] parameters.¶
The following parameters are introduced by this specification:¶
authorization_signing_alg_values_supportedalg values) supported by the authorization endpoint to sign the response.¶
authorization_encryption_alg_values_supportedalg values) supported by the authorization endpoint to encrypt the response.¶
authorization_encryption_enc_values_supportedenc values) supported by the authorization endpoint to encrypt the response.¶
Authorization servers SHOULD publish the supported response mode values utilizing the parameter response_modes_supported as defined in [RFC8414]. This document introduces the following possible values:¶
As JARM is used as a component in OAuth, many of the security considerations listed in OAuth 2.0 Security Best Current Practice [I-D.ietf-oauth-security-topics] apply. In addition, for the mechanisms described in this document, the following security considerations apply.¶
JWTs could be crafted to have an issuer that resolves to a JWK set URL with huge content, or is delivered very slowly, consuming server networking bandwidth and compute capacity. The resolved JWK set URL could also be used to DDoS targets on the web.¶
The client therefore MUST first check that the issuer of the JWT is well-known and expected for the particular authorization response before it uses this data to obtain the key needed to check the JWT's signature.¶
An OAuth protocol run is made of many distinct message exchanges between the client and server to complete the issuance of access and refresh tokens. Even if every message itself is integrity protected, it is still conceivable that one or more of the messages are exchanged with another message created for a different protocol run. The leakage and reuse of encrypted messages in Section 5.4 is an example of such problems. To mitigate this problem, it is considered good practice to implement additional protection provided by PKCE [RFC7636] as described in [I-D.ietf-oauth-security-topics].¶
Mix-up is an attack on scenarios where an OAuth client interacts with multiple authorization servers. The goal of the attack is to obtain an authorization code or an access token by tricking the client into sending those credentials to the attacker instead of using them at the respective endpoint at the authorization/resource server.¶
The JWT secured response mode enables clients to detect this attack by providing an identification of the sender (iss) and the intended audience of the authorization response (aud).¶
Authorization servers MAY encrypt the authorization response therewith providing a means to prevent leakage of authorization codes in the user agent (e.g. during transmission, in browser history or via referrer headers). Note, however, that the entire response is then potentially subject to leakage. An encrypted response doesn't remove the need for additional protections provided by mechanisms such as PKCE [RFC7636].¶
JARM only defines an alternative way of encoding the authorization response message and therefore does not materially impact the privacy considerations of OAuth 2.0, which is a complex and flexible framework with broad-ranging privacy implications.¶
The content of a conventional authorization response message (e.g., code and state) does not typically convey personally identifiable information (PII). However, using encrypted JARM may improve privacy by reducing the potential for inadvertent information disclosure in cases where the authorization response might contain PII (such as other response types or extensions).¶
This specification requests registration of the following client metadata definitions in the IANA "OAuth Dynamic Client Registration Metadata" registry established by [RFC7591]:¶
authorization_signed_response_alg¶
authorization_encrypted_response_alg¶
authorization_encrypted_response_enc¶
The following people contributed to this document:¶
Copyright (c) 2022 The OpenID Foundation.¶
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