High severity · published Sep 22, 2025
CVE-2025-59420
Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)
7.5
CVSS
0.3%
EPSS
No
On KEV
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 vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 18% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
AU
authlib · authlib
< 1.6.4 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-59420
CVE.org
https://www.cve.org/CVERecord?id=CVE-2025-59420
Patch
https://github.com/authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479df
Exploit
https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32
lists.debian.org
https://lists.debian.org/debian-lts-announce/2025/10/msg00032.html
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