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.

Affected: npm: fast-jwt

### 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 `allowed…