Security Digest · Vulnerability record
CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass
Authlib is a Python library which builds OAuth and OpenID Connect servers.
CVSS 9.1 Critical EPSS 0.5% · p44
Should you act?
Critical and remotely reachable
No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- 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.
From the GitHub advisory
Description
Summary
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 behavior violates RFC 7515 §4.1.3 and the validation algorithm defined in RFC 7515 §5.2.
Details
Weakness type: CWE-347.
Product: authlib (PyPI) · Assigned by: GitHub_M · Published: 2026-03-16
Affected and fixed versions
| Ecosystem | Package | Vulnerable | First fixed |
|---|---|---|---|
| PyPI | authlib | <= 1.6.8 | 1.6.9 |
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| authlib | authlib | < 1.6.9 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_M | 3.1 | 9.1 Critical | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
| redhat-SADP | 3.1 | 9.1 Critical | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
| GitHub | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: none
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- 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.
Timeline
- — CVE record published by GitHub_M
- — GitHub-reviewed advisory GHSA-wvwj-cvrp-7pv5 published (critical)
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
- access.redhat.com access.redhat.com
Vulnerability records
- GitHub Advisory GHSA-wvwj-cvrp-7pv5 github.com
- CVE.org record CVE-2026-27962 cve.org
- NVD entry CVE-2026-27962 nvd.nist.gov
Other references from the CVE record and advisory
- github.com github.com
- github.com github.com
- github.com github.com
- access.redhat.com access.redhat.com
- RHBZ#2448164 bugzilla.redhat.com
- security.access.redhat.com security.access.redhat.com