Encoding claims in the OAuth 2 state parameter using a JWT
Yubico
ve7jtb@ve7jtb.com
http://www.thread-safe.com/
torsten@lodderstedt.net
Ping Identity
hzandbelt@pingidentity.com
http://hanszandbelt.wordpress.com
JOSE
JSON Web Signature
JWS
JSON Web Encryption
JWE
JSON Web Key
JWK
JSON Web Algorithms
JWA
JWT
This draft provides a method for a client to encode one or more elements encoding
information about the session into the OAuth 2 state parameter.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
In the OAuth 2.0 Authorization protocol , the Authorization server SHOULD
perform an exact string comparison of the redirect_uri
parameter with the redirect_uri parameter registered by
by the client. This is essential for preventing token leakage to third parties in
the OAuth implicit flow.
As a result of this clients can not safely add extra query parameters to the
redirect_uri parameter that encode additional client state
information.
The Client MUST use the state parameter 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 JWT for use as the value of the state parameter.
The JWT may be sent without integrity protection, with integrity protection
JWS,
or with both integrity and confidentiality protection JWE.
The client is free
to choose the appropriate protection for its use-case as the state parameter is treated as opaque by the
Authorization Server (AS).
The OAuth Authorization request state parameter
consists of a JWT, optionally
signed with JWS or encrypted with
JWE, 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 parties.
RECOMMENDED if signed or encrypted. Identifier of the key used
to sign this state token at the issuer. Identifier of the key used
to encrypt this JWT state token at the issuer. This SHOULD be included in the
JWE header.
OPTIONAL. Timestamp of when this Authorization Request
was issued.
OPTIONAL. The exp (expiration time)
claim identifies the expiration time on or after which the
JWT MUST NOT be accepted for processing. The processing
of the exp claim requires that
the current date/time MUST be before the expiration
date/time listed in the exp
claim. Implementers MAY provide for some small leeway,
usually no more than a few minutes, to account for clock skew.
Its value MUST be a number containing an IntDate value.
Use of this claim is OPTIONAL. This is RECOMMENDED if
the JWT state token is being produced by the AS.
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.
RECOMMENDED. The jti (JWT ID) claim
provides a unique identifier for the JWT. The identifier
value MUST be assigned in a manner that ensures that there
is a negligible probability that the same value will be
accidentally assigned to a different data object. The
jti claim can be used to
prevent the JWT from being replayed.
The jti value is a case-sensitive string.
Use of this claim is OPTIONAL.
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.
This is REQUIRED if
the JWT state token is being produced by the AS and
issued with a access_token in the authorization
response.
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.
This is REQUIRED if
the JWT state token is being produced by the AS and
issued with a code in the authorization
response.
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 JWS
in such a way that it can verify the
signature. The JWA algorithm HS256 with a key of 256bits
is recommended.
The issuer MAY sign the JWT with JWS
using JWA algorithm none if integrity protecting the
contents of the state parameter is not required.
If the state parameter contains information the client
doesn'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 JWT. 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 parameter the client must validate its integrity.
The client parses it as a JWT. It then verifies the
signature of the JWT (if signed) using JWS.
The key used to sign the JWT
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 JWT
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 as claim if present MUST correspond to the
URI endpoint registered as the redirect_uri for
that AS.
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 be used as the value of rfp.
While OAuth strongly recommends that clients use TLS to secure their 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.
A client Authorization request might be redirected from the AS intended by the
client, as part of an attack to confuse the client, and cause it to deliver a
code or access tokens to a endpoint that the attacker can intercept them from.
A Client that has multiple client_id issued by more than one AS SHOULD register a
distinct redirect_uri value with each AS.
The redirect_uri
that the Authorization response is received on
MUST match the AS identified in the as
claim.
If the AS allows pattern matching of query paramaters in the
redirect_uri the identifier for the AS MUST
be contained in the URI path component.
Some information in the state JWT such as target_link_uri value for redirecting the user to
the application 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 logs, 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.
OAuth Signed Authorization request should be used to
integrity protect paramaters sent by the client to the authorization server.
The state paramater is intended for the consumption of the client, and to be
opaque to the AS.
Signed State should not be used to pass paramaters from the client to the AS.
[[ to be removed by the RFC editor before publication as an RFC
]]
-05
Added reference to OAuth Signed request
-04
Updated references to JOSE/JWT