OAuth2: What is the difference between the JWT Authorization Grant and Client Credentials Grant with JWT client authentication?

Question

The OAuth2 JWT Profile introduces the possibility to use JWTs both as authorization grant and as client authentication.

The JWT client authentication feature is independent of a certain grant type, and can be used with any grant type, also the client credentials grant.

However, using the JWT grant type seems to do exactly the same as using the client credentials grant with JWT client authentication, except that the syntax is slightly different.

In both cases the client contacts the token endpoint to get an access token:

POST /token.oauth2 HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=[JWT]

vs

POST /token.oauth2 HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&client_assertion=[JWT]

Answer 1

A slightly different perspective on the great answer by Josh C: as it happens both the client authentication and the grant credentials can be expressed as JWTs but the semantics behind them are different.

It is about separation of concerns: clients authenticate with a credential that identifies them i.e. they are the so-called subject whereas they use grants that were issued to them i.e. they are the so-called audience. Or as version 12 of the draft spec (https://datatracker.ietf.org/doc/html/draft-ietf-oauth-jwt-bearer-12) says:

2. The JWT MUST contain a "sub" (subject) claim identifying the principal that is the subject of the JWT. Two cases need to be differentiated:

   A. For the authorization grant, the subject typically identifies an authorized accessor for which the access token is being requested (i.e., the resource owner or an authorized delegate), but in some cases, may be a pseudonymous identifier or other value denoting an anonymous user.

   B. For client authentication, the subject MUST be the "client_id" of the OAuth client.

Answer 2

Probably very little. The way a client is identified and the way auth grants are requested are two different notions in OAuth, so the questions address those notions separately:

• Can a client authenticate with an authorization server using a JWT? Yes.
• Can a client make a grant request using a JWT? Yes.

The spec seems to hint that the separation is simply a design decision, deferring to policy makers to find what scenarios are valuable to them:

The use of a security token for client authentication is orthogonal to and separable from using a security token as an authorization grant. They can be used either in combination or separately. Client authentication using a JWT is nothing more than an alternative way for a client to authenticate to the token endpoint and must be used in conjunction with some grant type to form a complete and meaningful protocol request. JWT authorization grants may be used with or without client authentication or identification. Whether or not client authentication is needed in conjunction with a JWT authorization grant, as well as the supported types of client authentication, are policy decisions at the discretion of the authorization server.

One concrete possibility: The separation could allow for an authorization server to set up different policies along client types. For example, in the case of a public client (like a mobile app), the server should not accept the client creds grant type. Instead, the server could accept the JWT grant type for public clients and issue a token of lesser privilege.

Other than that, I would suppose that the design simply offers a degree of freedom for clients and servers to pivot around--keep this part of the existing handshake the same while migrating this part--as the need arises.