10.8. Cross-JWT Confusion

10.8. Cross-JWT Confusion

As described in Section 2.8 of [RFC8725], attackers may attempt to use a JWT issued for one purpose in a context that it was not intended for. The mitigations described for these attacks can be applied to Request Objects.

One way that an attacker might attempt to repurpose a Request Object is to try to use it as a client authentication JWT, as described in Section 2.2 of [RFC7523]. A simple way to prevent this is to never use the client ID as the sub value in a Request Object.

Another way to prevent cross-JWT confusion is to use explicit typing, as described in Section 3.11 of [RFC8725]. One would explicitly type a Request Object by including a typ Header Parameter with the value oauth-authz-req+jwt (which is registered in Section 9.4.1). Note, however, that requiring explicitly typed Request Objects at existing authorization servers will break most existing deployments, as existing clients are already commonly using untyped Request Objects, especially with OpenID Connect [OpenID.Core]. However, requiring explicit typing would be a good idea for new OAuth deployment profiles where compatibility with existing deployments is not a consideration.

Finally, yet another way to prevent cross-JWT confusion is to use a key management regime in which keys used to sign Request Objects are identifiably distinct from those used for other purposes. Then, if an adversary attempts to repurpose the Request Object in another context, a key mismatch will occur, thwarting the attack.