Vulnerability Description
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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Authlib | Authlib | < 1.6.4 |
Related Weaknesses (CWE)
References
- https://github.com/authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479Patch
- https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32ExploitVendor Advisory
- https://lists.debian.org/debian-lts-announce/2025/10/msg00032.html
FAQ
What is CVE-2025-59420?
CVE-2025-59420 is a vulnerability with a CVSS score of 7.5 (HIGH). 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), vio...
How severe is CVE-2025-59420?
CVE-2025-59420 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2025-59420?
Check the references section above for vendor advisories and patch information. Affected products include: Authlib Authlib.