CVE-2025-59420 HIGH

CVE-2025-59420: Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

Vendor Authlib
Product authlib
Weakness CWE-345
Published September 22, 2025
Last update November 3, 2025
View on NVD All CVEs

CVSS base score

7.5/10
Attack vector Network
Attack complexity Low
Privileges required None
User interaction None
Confidentiality None
Integrity High

CVSS vector

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

What the vulnerability does

01Description

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.

Key dates

02Disclosure timeline

September 22, 2025 CVE published
November 3, 2025 Record updated