# Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

Advisory record · `GHSA-9ggr-2464-2j32`

- **Identifiers:** `GHSA-9ggr-2464-2j32` · `CVE-2025-59420` · `PYSEC-2026-1200`
- **State:** Confirmed
- **Severity:** CVSS 7.5 High
- **Critical:** No
- **Weakness (CWE):** [CWE-345](https://cwe.mitre.org/data/definitions/345.html) · [CWE-863](https://cwe.mitre.org/data/definitions/863.html)
- **Tags:** [Broken access control](https://agentprime.io/advisories/tag/access-control.md) · [CWE-345](https://agentprime.io/advisories/tag/cwe-345.md)
- **Published:** 22 Sept 2025
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 22 Sept 2025, 14:42 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `authlib` (PyPI): affected `<1.6.4`; fixed in `1.6.4`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Authlib versions before 1.6.4 accept JWS tokens that list unknown critical header parameters in \`crit\`, which RFC 7515 says must be rejected. An attacker who can present a signed token to a service using Authlib's JsonWebSignature deserialization can get that token accepted where stricter verifiers would reject it, which may enable policy bypass, replay, or privilege escalation if \`crit\` carries security-relevant data. Upgrade the authlib package to version 1.6.4 or later.

Written 5 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.5 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0027, the 20th percentile, on 11 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:34 UTC: First seen by agentprime
- 31 Oct 2026, 14:34 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-9ggr-2464-2j32)

## Sources

- [GitHub Advisory Database: GHSA-9ggr-2464-2j32](https://github.com/advisories/GHSA-9ggr-2464-2j32)
- [OSV: GHSA-9ggr-2464-2j32](https://osv.dev/vulnerability/GHSA-9ggr-2464-2j32)
- <https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32>
- <https://nvd.nist.gov/vuln/detail/CVE-2025-59420>
- <https://github.com/authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479df>
- <https://github.com/authlib/authlib>
- <https://lists.debian.org/debian-lts-announce/2025/10/msg00032.html>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-9ggr-2464-2j32>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-9ggr-2464-2j32/>. Everything for agents: <https://agentprime.io/llms.txt>.