CVE-2025-59420
Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)
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.
How to fix CVE-2025-59420
To remediate CVE-2025-59420, upgrade the affected package to a fixed version below.
- —upgrade to 0.15.4-1+deb11u1 or later
- —upgrade to 1.6.4 or later
Is CVE-2025-59420 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (2)
- from 0, < 0.15.4-1+deb11u1
- from 0, < 1.6.4
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N |