CVE-2025-59420: Authlib

High severity, CVSS 7.5. EPSS: 0.3% chance of exploitation in the next 30 days.

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.4, Authlib’s JWS verification accepts tokens that declare unknown critical header parameters (crit), violating RFC 7515 “must‑understand” 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 mixed‑language fleets, this enables split‑brain verification and can lead to policy bypass, replay, or privilege escalation. This issue has been patched in version 1.6.4.

Affected products

• Authlib Authlib: before 1.6.4 (fixed in 1.6.4)

Published 2025-09-22. Last modified 2026-06-17.