CVE-2026-44351

The record

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.

Affected packages

Related vulnerabilities

• GHSA-gmvf-9v4p-v8jc fast-jwt: JWT auth bypass due to empty HMAC secret accepted by async key resolver Same vulnerability
• GHSA-cjw9-ghj4-fwxf fast-jwt has a ReDoS when using RegExp in allowed* leading to CPU exhaustion during token… Also affects fast-jwt
• GHSA-3j8v-cgw4-2g6q fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation… Also affects fast-jwt
• CVE-2026-35040 fast-jwt provides fast JSON Web Token (JWT) implementation. Also affects fast-jwt
• CVE-2026-35041 fast-jwt provides fast JSON Web Token (JWT) implementation. Also affects fast-jwt
• CVE-2026-35039 fast-jwt provides fast JSON Web Token (JWT) implementation. Also affects fast-jwt
• CVE-2026-35042 fast-jwt provides fast JSON Web Token (JWT) implementation. Also affects fast-jwt
• CVE-2026-34950 fast-jwt provides fast JSON Web Token (JWT) implementation. Also affects fast-jwt