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Vulnerability record · CVE-2025-59420 · published 22 September 2025

CVE-2025-59420: Authlib insufficient verification of data authenticity vulnerability

Authlib · Authlib

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.

7.5 CVSS 3.1 High EPSS 0.26% · top 83.8% CWE-345 · Insufficient verification of data authenticityCWE-863 · Incorrect authorization
7.5CVSS 3.1 base score
0.26%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References, 1 tagged exploit
17 Jun 2026Last modified by NVD

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

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479df Patch
https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32 ExploitVendor Advisory
https://lists.debian.org/debian-lts-announce/2025/10/msg00032.html

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2025-59420), CISA KEV, FIRST EPSS (scores of 2026-09-28). This page is refreshed as NVD updates the record.