Encoding claims in the OAuth 2 state paramater using a JWT

In the OAuth 2.0 Authorization protocol , the Authorization server SHOULD perform an exact string comparison of the redirect_uri paramater with the redirect_uri paramater registered by by the client. This is essential for stopping token leakage to third parties in the OAuth implicit flow. As a result of this clients can not safely add extra query paramaters to the redirect_uri paramater that encode additional client state information. The Client MUST use the state paramater to encode both Cross Site Request Forgery protection and any other state information it wishes to preserve for itself regarding the authorization request. This draft proposes a mechanism whereby multiple state attributes can be encoded into a JSON Web Token for use as the value of the state paramater. The JWT may be sent without integrity protection, with integrity protection , or with both integrity and confidentiality protection . The client is free to choose the appropriate protection for it's use-case as the state paramater is treated as opaque by the Authorization Server (AS).

The OAuth Authorization request state paramater consists of a , optionally signed with or encrypted with , whose payload contains claims as defined here. REQUIRED. string containing a verifiable identifier for the browser session, that cannot be guessed by a third party. The verification of this element by the client protects it from accepting authorization responses generated in response to forged requests generated by third parities. RECOMMENDED if signed. Identifier of the key used to sign this client identifier at the issuer. OPTIONAL. Timestamp of when this Authorization Request was issued. OPTIONAL. string identifying the party that issued this state value. OPTIONAL. string identifying the client that this state value is intended for. OPTIONAL. URI containing the location the user agent is to be redirected to after authorization. OPTIONAL. string identifying the authorization server that this request was sent to. OPTIONAL. Access Token hash value. Its value is the base64url encoding of the left-most half of the hash of the octets of the ASCII representation of the access_token value, where the hash algorithm used is the hash algorithm used in the alg parameter of the State Token's JWS header. For instance, if the alg is RS256, hash the access_token value with SHA-256, then take the left-most 128 bits and base64url encode them. The at_hash value is a case sensitive string. OPTIONAL. Code hash value. Its value is the base64url encoding of the left-most half of the hash of the octets of the ASCII representation of the code value, where the hash algorithm used is the hash algorithm used in the alg header parameter of the State Token's JWS header. For instance, if the alg is HS512, hash the code value with SHA-512, then take the left-most 256 bits and base64url encode them. The c_hash value is a case sensitive string. The issuer may add additional claims to the token. The producer and the consumer of the JWT are the same or closely related entities so collision resistance of claim names should not be a concern. The issuer SHOULD sign the JWT with in such a way that it can verify the signature. The algorithm HS256 with a key of 256bits is recommended. The issuer MAY sign the with algorithm none if integrity protecting the contents of the state paramater is not required. If the state paramater contains information the client dosen't want to disclose to the Authorization server or user, the issuer MAY encrypt the JWT with JWE. The JWA algorithm ("alg") of "dir" and encryption algorithm ("enc") of "A128CBC-HS256" are recommended for symmetric encryption. In the case of the state value being created by the Issuer the iss and aud claims MUST be included in the JWT. The jwt MUST also be signed with . If the State token is issued with a code c_hash MUST be included. If the State Token is issued with a Access Token at_hash MUST be included.

Upon receiving a state paramater the client must validate its integrity. The client parses it as a JWT. It then verifies the signature if the JWT (if signed) using . T he key used to sign the MAY be indicated by the kid field. The client MAY use other means to validate the JWT and determine its authenticity. The client then reads the fields inside the and uses these to configure the user experience and security parameters of the authorization. The rfp claim MUST be validated by the client by comparing it to the secret information that it used to create the rfp value.

The client MUST create a value that cannot be guessed by a third party attacker and used to forge requests. There are many possible ways to create this value. For reference two common ways will be listed. It is completely up to the purview of the particular client which generation methods, and which claims, they will accept.

Many clients that are web servers maintain session state for browsers in a server side store. These clients can generate a random value with sufficient entropy that an attacker cannot guess future values. This value can be stored in the server side store and used directly as the value of rfp.

Some clients that are web servers maintain session state for browsers using browser stored cookies or HTML5 local storage. These clients can generate a hash value based on a HTTPS: bound session cookie or other browser side information that is not accessible to third parties. This hash value can directly as the value of "xsrf". While OAuth strongly recommends that clients use TLS to secure there endpoints, if a client is not using TLS it MUST produce the value of rfp by using a HMAC algorithm with a secret known only to itself over the browser stored information.

Some clients may be willing to rely on the Authorization server providing protection for Cross Site Request Forgery. In Cases where the Authorization server and the client have a pre-established relationship, and the client is willing to accept flows initiated by the Authorization server, the string "iss" may be used as the value of rfp.

[ maybe we register the "rfp" claim above? ] This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC.

Some information in the state JWT such as target uri for redirecting the user to might have some security impact is the user modifies them intentionally or unintentionally. To prevent tampering with the "state" value the client may integrity protect the contents of the JWT. The client may have information that it wants to protect from disclosure to the Authorization server, in loggs. to proxies, or to the user. In this case encrypting the JWT as a JWE is required to protect the confidentiality of the state information.