CVE-2025-59420

Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

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

authlib

CVSS v3.1 GITHUB

7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

0.26% chance of exploitation in the next 30 days, 17th percentile

Published

2025-09-22

Updated

2025-11-03

Proof-of-concept exploits (1)

• advisories/GHSA-9ggr-2464-2j32

Security advisories

• authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32
• authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479df

References

• CVE Record
• NVD entry