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Advisory record · GHSA-9ggr-2464-2j32

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

CVSS 7.5 HighEPSS 0.0027 · 20th percentile

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.

PackageEcosystemAffectedFixed in
authlibPyPI<1.6.41.6.4
Identifiers
GHSA-9ggr-2464-2j32 · CVE-2025-59420 · PYSEC-2026-1200
Weakness (CWE)
CWE-345 · CWE-863
Tags
Published
22 Sept 2025
Updated
10 Sept 2026

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

ScoreRatingCVSSVectorSource
7.5 High 3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0027, the 20th percentile, on 10 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:34 UTC First seen by agentprime
  2. 31 Oct 2026, 14:34 UTC Confirmed by OSV

Sources

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.