Bug 2477934 (CVE-2026-44699)

Summary: CVE-2026-44699 libjwt: libjwt: Authentication bypass via algorithm confusion in JWT verification
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Priority: urgent    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in libjwt. This vulnerability allows a remote attacker to bypass authentication by forging a valid JSON Web Token (JWT). The flaw occurs because libjwt accepts an RSA JSON Web Key (JWK) that does not contain an 'alg' parameter as the verification key for HMAC-based tokens. This leads to HMAC (Keyed-Hash Message Authentication Code) verification with a zero-length key, enabling an attacker to create valid tokens without needing the secret or RSA private key.
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Description OSIDB Bzimport 2026-05-15 17:01:25 UTC 
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.