yes.com

torsten@lodderstedt.net

Ping Identity

bcampbell@pingidentity.com

Internet FAPI security oauth2 Warning This document is not an OIDF International Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an International Standard. Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation. Foreword The OpenID Foundation (OIDF) promotes, protects and nurtures the OpenID community and technologies. As a non-profit international standardizing body, it is comprised by over 160 participating entities (workgroup participants). The work of preparing implementer drafts and final international standards is carried out through OIDF workgroups in accordance with the OpenID Process. Participants interested in a subject for which a workgroup has been established has the right to be represented in that workgroup. International organizations, governmental and non-governmental, in liaison with OIDF, also take part in the work. OIDF collaborates closely with other standardizing bodies in the related fields. Final drafts adopted by the Workgroup through consensus are circulated publicly for the public review for 60 days and for the OIDF members for voting. Publication as an OIDF Standard requires approval by at least 50 % of the members casting a vote. There is a possibility that some of the elements of this document may be the subject to patent rights. OIDF shall not be held responsible for identifying any or all such patent rights.

Introduction 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.

Notational 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 when, and only when, they appear in all capitals, as shown here.

Terms and definitions For the purpose of this document, the terms defined in and apply.

Symbols and Abbreviated terms API – Application Programming Interface CSRF - Cross Site Request Forgery HTTP – Hyper Text Transfer Protocol OIDF - OpenID Foundation TLS – Transport Layer Security

JWT-based Response Mode This document defines a new JWT-based mode to encode OAuth 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 corresponding to these techniques.

The JWT Response Document 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 . 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 , will also be added to the JWT. The JWT response document MAY contain further element, e.g. the claims defined in . Implementation SHOULD adhere to the respective processing rules and ignore unrecognized elements.

Example Response Type "code" For the grant type authorization "code" the JWT contains the response parameters as defined in , 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 , 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" }

Signing and Encryption 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 . 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 ). 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 .

Response Encoding This document defines the following response mode values:

• query.jwt
• fragment.jwt
• form_post.jwt
• jwt

Response Mode "query.jwt" 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 . 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.

Response Mode "fragment.jwt" 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 . 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

Response Mode "form_post.jwt" The response mode "form_post.jwt" uses the technique described in 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

Response Mode "jwt" 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 , except "none", is "fragment.jwt".

Processing rules 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:

1. (OPTIONAL) The client decrypts the JWT using the default key for the respective issuer or, if applicable, determined by the 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 ).
2. The client obtains the 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.
3. The client obtains the 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.
4. The client checks the JWT's exp element to determine if the JWT is still valid. If the check fails, the client MUST abort processing and refuse the response.
5. The client MUST check the signature of the JWT according to and the algorithm 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 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 or SHOULD be utilized. The client MUST NOT process the grant type specific authorization response parameters before all checks succeed.

Client Metadata The Dynamic Client Registration Protocol defines an API for dynamically registering OAuth 2.0 client metadata with authorization servers. The metadata defined by , 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_alg

The JWS alg 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_alg

The JWE alg 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 . The default, if omitted, is that no encryption is performed.

authorization_encrypted_response_enc

The JWE enc 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 Server Metadata 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 parameters. The following parameters are introduced by this specification:

authorization_signing_alg_values_supported

OPTIONAL. A JSON array containing a list of the JWS signing algorithms (alg values) supported by the authorization endpoint to sign the response.

authorization_encryption_alg_values_supported

OPTIONAL. A JSON array containing a list of the JWE encryption algorithms (alg values) supported by the authorization endpoint to encrypt the response.

authorization_encryption_enc_values_supported

OPTIONAL. A JSON array containing a list of the JWE encryption algorithms (enc 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 . This document introduces the following possible values:

• query.jwt
• fragment.jwt
• form_post.jwt
• jwt

Security Considerations As JARM is used as a component in OAuth, many of the security considerations listed in OAuth 2.0 Security Best Current Practice apply. In addition, for the mechanisms described in this document, the following security considerations apply.

DoS using specially crafted JWTs 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.

Protocol Run Integrity 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 is an example of such problems. To mitigate this problem, it is considered good practice to implement additional protection provided by PKCE as described in .

Mix-Up 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).

Code Leakage 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 .

Privacy Considerations 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).

IANA Considerations

OAuth Dynamic Client Registration Metadata Registration This specification requests registration of the following client metadata definitions in the IANA "OAuth Dynamic Client Registration Metadata" registry established by :

Registry Contents

• Client Metadata Name: authorization_signed_response_alg
• Client Metadata Description: String value indicating the client's desired introspection response signing algorithm.
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]
• Client Metadata Name: authorization_encrypted_response_alg
• Client Metadata Description: String value specifying the desired introspection response encryption algorithm (alg value).
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]
• Client Metadata Name: authorization_encrypted_response_enc
• Client Metadata Description: String value specifying the desired introspection response encryption algorithm (enc value).
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]

OAuth Authorization Server Metadata Registration This specification requests registration of the following value in the IANA "OAuth Authorization Server Metadata" registry established by .

Registry Contents

• Metadata Name: authorization_signing_alg_values_supported
• Metadata Description: JSON array containing a list of algorithms supported by the authorization server for introspection response signing.
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]
• Metadata Name: authorization_encryption_alg_values_supported
• Metadata Description: JSON array containing a list of algorithms supported by the authorization server for introspection response encryption (alg value).
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]
• Metadata Name: authorization_encryption_enc_values_supported
• Metadata Description: JSON array containing a list of algorithms supported by the authorization server for introspection response encryption (enc value).
• Change Controller: IESG
• Specification Document(s): of [[ this specification ]]

Normative References Microsoft Illumila Google Google Facebook Microsoft Informative References NRI Ping Identity Microsoft Illumila NRI Ping Identity Microsoft Google Salesforce Google Microsoft NAT.Consulting Yubico Microsoft

Acknowledgements The following people contributed to this document:

• Torsten Lodderstedt (YES), Editor
• Brian Campbell (Ping Identity), Co-editor
• Nat Sakimura (NAT Consulting LLC) -- Chair
• Dave Tonge (Momentum Financial Technology) -- Chair
• Joseph Heenan (Authlete)
• Ralph Bragg (Raidiam)
• Vladimir Dzhuvinov (Connect2ID)
• Michael Schwartz (Gluu)
• Filip Skokan (Auth0|Okta)

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