Is there a public exploit for CVE-2025-59420?

No public exploit for CVE-2025-59420 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2025-59420?

CVE-2025-59420 scores CVSS 7.5 (v3.1), rated high. EPSS puts the chance of exploitation in the next 30 days at 0.28%.

What does CVE-2025-59420 affect?

CVE-2025-59420 affects authlib authlib.

7.5 High

Authlib is a Python library which builds OAuth and OpenID Connect servers.

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.

Assessment

CVSS: 7.5 (v3.1)
Severity: High
EPSS: 0.28% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 22 September 2025
Last updated: 19 October 2026
Affected: authlib · authlib
Weakness: CWE-345, CWE-863
Assigner: security-advisories@github.com

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

At the source

References

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