express-demo

jwks-rsa - Express Example

The jwks-rsa library provides a small helper that makes it easy to configure express-jwt with the RS256 algorithm. Using expressJwtSecret you can generate a secret provider that will provide the right signing key to express-jwt based on the kid in the JWT header.

const Express = require('express');
const { expressjwt: jwt } = require('express-jwt');
const jwksRsa = require('jwks-rsa');

...

// Initialize the app.
const app = new Express();
app.use(jwt({
  // Dynamically provide a signing key based on the kid in the header and the signing keys provided by the JWKS endpoint.
  secret: jwksRsa.expressJwtSecret({
    cache: true,
    rateLimit: true,
    jwksRequestsPerMinute: 5,
    jwksUri: `https://my-authz-server/.well-known/jwks.json`
  }),

  // Validate the audience and the issuer.
  audience: 'urn:my-resource-server',
  issuer: 'https://my-authz-server/',
  algorithms: [ 'RS256' ]
}));

By default express will return key or secret has to be provided if there's no match between the token and one of the signing certificates. To overwrite this behavior and inject your own error you can specify the following option:

jwksRsa.expressJwtSecret({
  ...,
  handleSigningKeyError: (err, cb) => {
    if (err instanceof jwksRsa.SigningKeyNotFoundError) {
      return cb(new Error('This is bad'));
    }

    return cb(err);
  }
});

Running the sample

DEBUG=express,jwks JWKS_HOST=https://my-authz-server AUDIENCE=urn:my-resource-server ISSUER=https://my-authz-server/ node server.js

Tip: You can use Auth0 to test this.

How does this work?

When you have the sample running you'll need to get a token from your Authorization Server, which will look like this:

eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImtpZCI6IlJrSTVNakk1T1VZNU9EYzFOMFE0UXpNME9VWXpOa1ZHTVRKRE9VRXpRa0ZDT1RVM05qRTJSZyJ9.eyJpc3MiOiJodHRwczovL3NhbmRyaW5vLmF1dGgwLmNvbS8iLCJzdWIiOiJhdXRoMHw1NjMyNTAxZjQ2OGYwZjE3NTZmNGNhYjAiLCJhdWQiOiJQN2JhQnRTc3JmQlhPY3A5bHlsMUZEZVh0ZmFKUzRyViIsImV4cCI6MTQ2ODk2NDkyNiwiaWF0IjoxNDY4OTI4OTI2fQ.NaNeRSDCNu522u4hcVhV65plQOiGPStgSzVW4vR0liZYQBlZ_3OKqCmHXsu28NwVHW7_KfVgOz4m3BK6eMDZk50dAKf9LQzHhiG8acZLzm5bNMU3iobSAJdRhweRht544ZJkzJ-scS1fyI4gaPS5aD3SaLRYWR0Xsb6N1HU86trnbn-XSYSspNqzIUeJjduEpPwC53V8E2r1WZXbqEHwM9_BGEeNTQ8X9NqCUvbQtnylgYR3mfJRL14JsCWNFmmamgNNHAI0uAJo84mu_03I25eVuCK0VYStLPd0XFEyMVFpk48Bg9KNWLMZ7OUGTB_uv_1u19wKYtqeTbt9m1YcPMQ

If you then decode this token (using jwt.io), you'll see the following header:

{
  "typ": "JWT",
  "alg": "RS256",
  "kid": "RkI5MjI5OUY5ODc1N0Q4QzM0OUYzNkVGMTJDOUEzQkFCOTU3NjE2Rg"
}

Using this kid we will try to find the right signing key in the signing keys provided by the JWKS endpoint you configured.

You can then call the sample application like this:

var request = require("request");

var options = {
  method: 'GET',
  url: 'http://localhost:4001/me',
  headers: { authorization: 'Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImtpZCI...' }
};

request(options, function (error, response, body) {
  if (error) throw new Error(error);

  console.log(body);
});

A few things will happen now:

1. express-jwt will decode the token and pass the request, the header and the payload to jwksRsa.expressJwtSecret
2. jwks-rsa will then download all signing keys from the JWKS endpoint and see if a one of the signing keys matches the kid in the header of the JWT. a. If none of the signing keys match the incoming kid, an error will be thrown b. If we have a match, we will pass the right signing key to express-jwt
3. express-jwt will the continue its own logic to validate the signature of the token, the expiration, audience, issuer, ...

If you repeat this call a few times you'll see in the console output that we're not calling the JWKS endpoint anymore, because caching has been enabled.

If you then make multiple calls with a kid that is not defined in the JWKS endpoint, you'll see that rate limiting will kick in.