GHSA-gmvf-9v4p-v8jc CRITICAL npm/fast-jwt

CVE: CVE-2026-44351

Summary

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. Reproducible 100% against the current latest release fast-jwt@6.2.3.

Preconditions

For this issue to occur the following MUST ALL be true:

1. The application developer (library consumer) uses an asynchronous callback function to set the key (e.g. createVerifier({key: async (decoded) => ... }))

2. The response from the async callback MUST return an empty string '' OR zero-length buffer (e.g. Buffer.alloc(0)). Any other empty/missing return values (e.g. null, undefined) do not trigger this issue

3. The library configuration must allow HMAC signatures. This is the default for the library.

4. The bad actor MUST have signed their token with an empty string. This is a trivial task and requires no special knowledge.

5. All other aspects of the token (e.g. EXP, IAT claims) MUST be valid. This issue ONLY affects signature checking and all other checks remain enforced.

Details

src/verifier.js prepareKeyOrSecret (lines 33-39):

function prepareKeyOrSecret(key, isSecret) {
  if (typeof key === 'string') {
    key = Buffer.from(key, 'utf-8')
  }
  return isSecret ? createSecretKey(key) : createPublicKey(key)   // ← no length check
}

src/verifier.js async key-resolver flow (lines 42

📌 来源: GitHub-Advisory | 🆔 CVE-2026-44351 | 📅 2026-05-06