Authlib versions from 1.3.1 up to but not including 1.9.0 contain a vulnerability in the handling of HMAC keys in the jose module. The check that prevents asymmetric public keys from being used as HMAC secrets only inspects the start of the key bytes, but the asymmetric key loader skips leading bytes before the PEM header. This discrepancy allows an attacker with only the issuer's public key to craft a valid HS256 token with arbitrary claims that will be accepted as authentic. The issue is an incomplete fix of a previous vulnerability (CVE-2024-37568) and affects the documented usage of the jwt.decode method. A fix is available in version 1.9.0.