fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set

[ Why it matters ]Priority signals

• Public proof-of-concept exploit exists (CISA SSVC)

[ Description ]Official description

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.1, fast-jwt createVerifier accepts an unsigned JWT when key is an empty string or null and algorithms is a non-empty allowlist. Falsy synchronous keys bypass prepareKeyOrSecret, allowedAlgorithms remains active, hasKey is false, and the empty signature avoids the verifySignature gate. An attacker can therefore submit a token containing arbitrary claims without possessing a signing key, resulting in authentication or authorization bypass. Claim validators still run, and non-empty keys, an empty key without algorithms, and the async key resolver path do not have this behavior. This issue is fixed in version 6.3.1.

[ Fixes ]Fixes and patches

• fast-jwt 6.3.1Fixed version · GitHub Advisories · npm package fast-jwt: upgrade to 6.3.1 or later (affected <= 6.3.0)

[ Packages ]Affected open-source packages

• fast-jwt npmAffected <= 6.3.0 · fixed in 6.3.1 · GHSA-8wpc-h4q6-8fxv

[ References ]References

• github.com/nearform/fast-jwt/commit/e22a151e83bf6d5e54e281216982d204d5665534
• github.com/nearform/fast-jwt/pull/649
• github.com/nearform/fast-jwt/releases/tag/v6.3.1
• github.com/nearform/fast-jwt/security/advisories/GHSA-8wpc-h4q6-8fxv