fast-jwt: Empty HMAC secret accepted via async key resolver - JWT auth bypass

Summary

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.

Timeline

1. May 13, 2026, 7:12 PM UTCCVE published
2. May 14, 2026, 6:13 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: total

3. May 14, 2026, 6:13 PM UTCRecord last updated

Affected versions

• nearform fast-jwt

  fixed in 6.2.4

• fast-jwtnpm

  fixed in 6.2.4

References 2