Configuring Your Express Server for Mutual TLS

How to use TLS, client authentication, and CA certificates in Express

Create a private key and request a certificate for your Express server

Before you can teach your server to speak TLS, you will need a certificate issued by a trusted certificate authority (CA). If your organization already runs its own CA and you have a private key and certificate for your Express server, along with your CA's root certificate, you can skip to the next step.

If your organization does not yet run its own internal CA, you can read more about creating and running a CA using the open source smallstep software here.

To get up and running quickly, we recommend creating a free hosted smallstep Certificate Manager authority.

To request a certificate from your CA using the step CLI, bootstrap your CA with step ca bootstrap and run the following command (sub the server name for the actual name / DNS name of your Express server).

$ step ca certificate "myserver.internal.net" server.crt server.key

Your certificate and private key will be saved in server.crt and server.key respectively.

Request a copy of your CA root certificate, which will be used to make sure each application can trust certificates presented by other applications.

$ step ca root ca.crt

Your certificate will be saved in ca.crt.

Configure Express to authenticate itself with its TLS certificate

We now want to instruct our Express server to identify itself using the certificate issued in the last step and to force clients to connect over TLS.

Using the https module (instead of app.listen()) to start your server, specify the locations of the server's certificate and private key.

const fs = require('fs');
const https = require('https');
const express = require('express');

const app = express();

app.get('/', (req, res) => {
  return res.send('Hello, world!');
});

https
  .createServer(
    {
      // ...
      cert: fs.readFileSync('server.crt'),
      key: fs.readFileSync('server.key'),
      // ...
    },
    app
  )
  .listen(9443);

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Configure Express to require clients to authenticate with a certificate issued by your CA

To tell Express to use mutual TLS and not just one-way TLS, we must instruct it to require client authentication to ensure clients present a certificate from our CA when they connect.

Using the https module (instead of app.listen()) to start your server, specify the location of your CA root certificate to use for authenticating client certificates.

In this case, we instruct our server to request client certificates, but not to reject unauthorized requests so that we can check for authorization later and provide a friendly message on client authentication failures.

const fs = require('fs');
const https = require('https');
const express = require('express');

const app = express();

app.get('/', (req, res) => {
  if (!req.client.authorized) {
    return res.status(401).send('Invalid client certificate authentication.');
  }

  return res.send('Hello, world!');
});

https
  .createServer(
    {
      // ...
      requestCert: true,
      rejectUnauthorized: false,
      ca: fs.readFileSync('ca.crt'),
      // ...
    },
    app
  )
  .listen(9443);

It's also possible to turn the above into an express middleware to authenticate on every request:

// ...

const clientAuthMiddleware = () => (req, res, next) => {
  if (!req.client.authorized) {
    return res.status(401).send('Invalid client certificate authentication.');
  }
  return next();
};

const app = express();
app.use(clientAuthMiddleware());

// ...

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That's it! Express should now be able to receive TLS connections from clients who authenticate themselves using a certificate issued by your trusted CA.

Automate certificate renewal

By default, step-ca issues certificates with a 24 hour expiration. Short-lived certificates have many benefits but also require that you renew your certificates each day before they expire. How you renew certificates is often dependent on how you deploy your application. See the step-ca certificate lifecycle management docs for more information.

All documentation content from the Hello mTLS project is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0).