# CVE-2025-59420: python-authlib: Authlib is a Python library which builds OAuth and OpenID Connect servers

CVE-2025-59420 is fixed in 2 products on 1 platform, first by DEBIAN-CVE-2025-59420, released September 22, 2025. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2025-59420
- Priority: High
- CVSS: 7.5 (high)
- EPSS: 0.0028
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 22, 2025

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | python-authlib Debian 12.15 (bookworm) | 1.2.0-1+deb12u1 | [DEBIAN-CVE-2025-59420](https://patchblox.com/security/data/fix/linux/debian-cve-2025-59420.md) | Sep 22, 2025 |
| Linux | python-authlib Debian 13.5 (trixie) | 1.6.0-1+deb13u1 | [DEBIAN-CVE-2025-59420](https://patchblox.com/security/data/fix/linux/debian-cve-2025-59420.md) | Sep 22, 2025 |
| Linux | python-authlib Debian 14 | 1.6.4-1 | [DEBIAN-CVE-2025-59420](https://patchblox.com/security/data/fix/linux/debian-cve-2025-59420.md) | Sep 22, 2025 |
| Linux | python3-authlib Ubuntu 22.04.1 LTS (jammy) | 0.15.5-1ubuntu0.1~esm1 | [python3-authlib 0.15.5-1ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-python3-authlib-0.15.5-1ubuntu0.1-esm1.md) | Oct 22, 2025 |
| Linux | python3-authlib Ubuntu 24.04.5 LTS (noble) | 1.3.0-1ubuntu0.1~esm1 | [python3-authlib 1.3.0-1ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-noble-python3-authlib-1.3.0-1ubuntu0.1-esm1.md) | Jan 8, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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