CVE-2025-59420

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.4, Authlibs JWS verification accepts tokens that declare unknown critical header parameters (crit), violating RFC 7515 mustunderstand semantics. An attacker can craft a signed token with a critical header (for example, bork or cnf) that strict verifiers reject but Authlib accepts. In mixedlanguage fleets, this enables splitbrain verification and can lead to policy bypass, replay, or privilege escalation. This issue has been patched in version 1.6.4.