Auth bypass in Authlib
CVE-2025-59420
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.
EPSS: 0.002 (15.8th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
Affected products
- Authlib — versions < 1.6.4
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- af854a3a-2127-422b-91ae-364da2661108
Frequently asked questions
- What is CVE-2025-59420?
- CVE-2025-59420 is a high-severity vulnerability in Authlib, classified under Insufficient Verification of Data Authenticity. CVSS score: 7.5/10. Published 2025-09-22.
- How severe is CVE-2025-59420?
- High severity. CVSS v3 base score is 7.5 out of 10.