---
title: "CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass"
url: https://erroneo.us/security/cve/CVE-2026-27962/
description: "CVE-2026-27962 (authlib (PyPI)): Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a JWK Header Injection…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-03-16
date_modified: 2026-09-10
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-27962"
kev: false
cvss: 9.1
epss: 0.00537
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.5% , percentile 44: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 44% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes authlib (PyPI) <= 1.6.8, fixed in 1.6.9 . Check your lockfile, including transitive dependencies.

## What it is

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a JWK Header Injection vulnerability in authlib's JWS implementation allows an unauthenticated attacker to forge arbitrary JWT tokens that pass signature verification. When key=None is passed to any JWS deserialization function, the library extracts and uses the cryptographic key embedded in the attacker-controlled JWT jwk header field. An attacker can sign a token with their own private key, embed the matching public key in the header, and have the server accept the forged token as cryptographically valid — bypassing authentication and authorization entirely. This issue has been patched in version 1.6.9.

## Affected packages

- PyPI `authlib` <= 1.6.8 → fixed: 1.6.9

## Affected products (CVE record)

- authlib authlib: < 1.6.9

## Severity

- GitHub_M CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- redhat-SADP CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.54% probability of exploitation activity in the next 30 days; percentile 44.

## Sources

- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:5665)
- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:7314)
- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:24853)
- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:19375)
- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:48085)
- [GitHub Advisory GHSA-wvwj-cvrp-7pv5](https://github.com/advisories/GHSA-wvwj-cvrp-7pv5)
- [CVE.org record CVE-2026-27962](https://www.cve.org/CVERecord?id=CVE-2026-27962)
- [NVD entry CVE-2026-27962](https://nvd.nist.gov/vuln/detail/CVE-2026-27962)
- [github.com](https://github.com/authlib/authlib/security/advisories/GHSA-wvwj-cvrp-7pv5)
- [github.com](https://github.com/authlib/authlib/commit/a5d4b2d4c9e46bfa11c82f85fdc2bcc0b50ae681)
- [github.com](https://github.com/authlib/authlib/releases/tag/v1.6.9)
- [access.redhat.com](https://access.redhat.com/security/cve/CVE-2026-27962)
- [RHBZ#2448164](https://bugzilla.redhat.com/show_bug.cgi?id=2448164)
- [security.access.redhat.com](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27962.json)

---
Source page: https://erroneo.us/security/cve/CVE-2026-27962/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt