In today’s digital world, security is a top priority for businesses. Whether you’re a Fortune 500 company or a startup just taking off, it’s essential to implement security measures in order to protect sensitive information. Security starts inside an organization; it starts with having Zero Trust principles that protect access to resources.

Mutual TLS (mTLS) is useful in a Zero Trust world to secure a wide range of network services and applications: APIs, web applications, microservices, databases and IoT devices. Cloudflare has products that enforce mTLS: API Shield uses it to secure API endpoints and Cloudflare Access uses it to secure applications. Now, with mTLS support for Workers you can use Workers to authenticate to services secured by mTLS directly. mTLS for Workers is now generally available for all Workers customers!

A recap on how TLS works

Before diving into mTLS, let’s first understand what TLS (Transport Layer Security) is. Any website that uses HTTPS, like the one you’re reading this blog on, uses TLS encryption. TLS is used to create private communications on the Internet – it gives users assurance that the website you’re connecting to is legitimate and any information passed to it is encrypted.

TLS is enforced through a TLS handshake where the client and server exchange messages in order to create a secure connection. The handshake consists of several steps outlined below:

The client and server exchange cryptographic keys, the client authenticates the server's identity and the client and server generate a secret key that’s used to create an encrypted connection.

For most connections over the public Internet TLS is sufficient. If you have a website, for example a landing page, storefront or documentation, you want any user to be able to navigate to it – validating the identity of the client isn’t necessary. But, if you have services that run on a corporate network or private/sensitive applications you may want access to be locked down to only authorized clients.

Enter mTLS

With mTLS, both the client and server present a digital certificate during the TLS handshake to mutually verify each other’s credentials and prove identities. mTLS adds additional steps to the TLS handshake in order to authenticate the client as well as the server.

In comparison to TLS, with mTLS the server also sends a ‘Certificate Request’ message that contains a list of parties that it trusts and it tells the client to respond with its certificate. The mTLS authentication process is outlined below:

mTLS is typically used when a managed list of clients (eg. users, devices) need access to a server. It uses cryptographically signed certificates for authentication, which are harder to spoof than passwords or tokens. Some common use cases include: communications between APIs or microservices, database connections from authorized hosts, and machine-to-machine IoT connections.

Introducing mTLS on Workers

With mTLS support on Workers, your Workers can now communicate with resources that enforce an mTLS connection. mTLS through Workers can be configured in just a few easy steps:

1. Upload a client certificate and private key obtained from your service that enforces mTLS using wrangler

wrangler mtls-certificate upload --cert cert.pem --key key.pem --name my-client-cert

2. Add a mtls_certificates binding in your project’s wrangler.toml file. The CERTIFICATE_ID is returned after your certificate is uploaded in step 1

mtls_certificates = [
 { binding = "MY_CERT", certificate_id = "<CERTIFICATE_ID>" }
]

3. Add the mTLS binding to any fetch() requests. This will present the client certificate when establishing the TLS connection.

index.js

export default {
   async fetch(request, environment) {
       return await environment.MY_CERT.fetch("https://example-secure-origin.com")
   }

}

To get per-request granularity, you can configure multiple mTLS certificates if you’re connecting to multiple hosts within the same Worker. There’s a limit of 1,000 certificates that can be uploaded per account. Contact your account team or reach out through the Cloudflare Developer Discord if you need more certificates.

Try it yourself

It’s that easy! For more information and to try it yourself, refer to our developer documentation – client authentication with mTLS.

We love to see what you’re developing on Workers. Join our Discord community and show us what you’re building!