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40Track*

Authlib JWS JWK Header Injection: Signature Verification Bypass

authlib · CVE-2026-27962 GHSA-WVWJ-CVRP-7PV5
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Assessment: Track*

SSVC decision points: exploitation poc, automatable yes, technical impact total, deployment low.

What is it?

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.

Evidence

No coverage recorded yet.

Exploitation

  • Exploit code published (not verified by us)
  • Exploited in the wild (confirmed by a source)
  • Listed as known exploited (CISA KEV)
CVSS
9.1
EPSS
–
First seen
16 Mar 2026
Status
corroborated

All authlib vulnerabilities