Warning about possible security issue

Hello,

As you all know, JWT provides different way to encode and decode a token. Some of those algorithm are asymmetric and others symmetric. The fact that you can have both of them working together create a breach if you can access the public_key.

I’d like to know what you think about the idea of having a warning, or not allowing it at all, to have both a symmetric and asymmetric algorithm in the jwt.decode function.

Here you can find an example of case where you might have a security issue with your JWT.

Exemple:

Actor 1:

1. Request a JWT from the Auth server
2. Contact App A with the JWT provided by Auth Server.
3. App A will validate the JWT with the public key

Actor 2:

1. Request a JWT from the Auth server
2. Get the content and modify it
3. generate a new JWT token signed with the public key and the algorithm “HS256”
4. Contact the App A with the new JWT
5. Because both Symmetric (HS256) and Asymmetric (RS256) are allowed together in App A, the new JWT is validated and accepted

[nigoroll]

OK, I understand your point now. A public key argument does not make sense for HS*

I agree that the interface could be improved, see also https://github.com/jpadilla/pyjwt/issues/408#issuecomment-586696809

[nigoroll]

Making public a shared secret used for HS* is wrong.

Consequently, re-using a (published) public key for HS* is wrong.

Bluntly, I see no point in your scenario.