GMS-2024-212: commonground-api-common unexploitable privilege escalation in JWT authentication middleware

Impact

This is a privilege escalation vulnerability. The impact is negligible and entirely theoretical.

A non-exploitable weakness was found in how the client-supplied JWTs are verified. Because an explicit allow-list of known algorithms is used in the PyJWT library, user-supplied (invalid) algorithms are rejected.

If this was not the case, then the client JWTs could be tampered with, resulting in privilege escalation which would allow the attacker to perform any operation as any client (impersonation) without leaving a trace of the real user/client.

Patches

Will be fixed in 1.12.2

Workarounds

None needed. But be careful when updating PyJWT. Check that the used PyJWT has no algorithms specified with a name in “”, “HS25”, “HS2”, “HS”, “H”, or that those algorithms are acceptable.

Details

The header and payload of JSON Web Tokens (JWTs) are cryptographically signed with an algorithm. A JWT has a header field alg that specifies the algorithm used in the signature.

The vng-api-common.middleware.AuthMiddleware uses PyJWT to check the validity of JWT and indicates it should be “HS256”, otherwise an attacker could construct a token with a cryptographically weak token. It should indicate this with a list of acceptable algorithms ["HS256"], but instead the string "HS256" is passed to PyJWT. PyJWT does not check the type of the argument and checks if the alg string in the header exists in the acceptable algorithms value with the in operator. Any substring of "HS256" passes this in check. It is not exploitable because there is no such substring in de set of algorithms PyJWT supports.