security: verify JWT signature in OAuth2 base class (CWE-347) - #1632

Summary

The get_decoded_jwt_token() method in OAuth2ClientBase decodes JWT tokens with verify=False and options={"verify_signature": False}, allowing an attacker to forge tokens with arbitrary claims.

Security Impact

CWE-347: Improper Verification of Cryptographic Signature

This is a critical authentication bypass vulnerability. The unverified JWT decode is called from:

• permissions.py:171 — get_user_from_oauth2() uses get_decoded_jwt_token() to extract the sub claim and look up the authenticated user
• permissions.py:82,89,109 — id_token_has_expired() calls get_decoded_jwt_token() to check token expiry

An attacker can craft a JWT with an arbitrary sub claim (pointing to any user) and an exp claim far in the future. Since the signature is never verified, the application will accept the forged token and authenticate the attacker as any user.

Note: The KeycloakDiffgramClient.verify_token() already properly verifies JWT signatures using the provider's public key — this fix brings the same verification to the base class method used in the permissions flow.

Fix

When OAUTH2_PROVIDER_PUBLIC_KEY is configured:

• Verify the JWT signature using the provider's RSA public key
• Return an empty dict on verification failure (safe default)

When no public key is configured:

• Log a clear security warning to alert operators
• Fall back to unverified decode (preserves backward compatibility)

Also removes the deprecated verify parameter (replaced by options dict in PyJWT >= 2.0).

Testing

• Existing JWT decode behavior is preserved when public key is configured
• Invalid/forged tokens now correctly fail verification
• Backward compatible with deployments that haven't configured the public key yet (with warning)