Monitor vulnerabilities like this one. Sign up free to get alerted when software you use is affected.

CVE-2026-44699: LibJWT: Accepts Invalid RSA Key for Some Token Types

CVE-2026-44699

Summary

LibJWT, a library used for JSON Web Tokens, has a flaw in versions 3.0.0 to 3.3.2. This can allow an attacker to create fake tokens without knowing the secret key. If you're using LibJWT, update to version 3.3.3 or later to fix the issue.

Original title

Original description

LibJWT is a C JSON Web Token Library. From 3.0.0 to 3.3.2, libjwt accepts an RSA JWK that does not contain an alg parameter as the verification key for an HS256/HS384/HS512 token. In the OpenSSL backend, this causes HMAC verification to run with a zero-length key, so an attacker can forge a valid JWT without knowing any secret or RSA private key. This is an algorithm-confusion authentication bypass. It affects applications that load RSA keys from JWKS where alg is omitted, which is valid JWK syntax and common in real deployments, and then choose the verification algorithm from the JWT header, for example in a kid lookup callback. This vulnerability is fixed in 3.3.3.

nvd CVSS4.0 9.1

Vulnerability type

CWE-327 Use of a Broken Cryptographic Algorithm

CWE-347 Improper Verification of Cryptographic Signature

https://github.com/benmcollins/libjwt/security/advisories/GHSA-q843-6q5f-w55g

Published: 15 May 2026 · Updated: 28 May 2026 · First seen: 15 May 2026