mTLS

When using HTTPS ↗, a server presents a certificate for the client to authenticate in order to prove their identity. For even tighter security, some services require that the client also present a certificate.

This process - known as mTLS ↗ - moves authentication to the protocol of TLS, rather than managing it in application code. Connections from unauthorized clients are rejected during the TLS handshake instead.

To present a client certificate when communicating with a service, create a mTLS certificate binding in your Worker project’s wrangler.toml file. This will allow your Worker to present a client certificate to a service on your behalf.

Warning

Currently, mTLS for Workers cannot be used for requests made to a service that is a proxied zone on Cloudflare. If your Worker presents a client certificate to a service proxied by Cloudflare, Cloudflare will return a 520 error.

First, upload a certificate and its private key to your account using the wrangler mtls-certificate command:

Warning

The wrangler mtls-certificate upload command requires the SSL and Certificates Edit API token scope. If you are using the OAuth flow triggered by wrangler login, the correct scope is set automatically. If you are using API tokens, refer to Create an API token ↗ to set the right scope for your API token.

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

Then, update your Worker project’s wrangler.toml file to create an mTLS certificate binding:

wrangler.toml

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

Note

Certificate IDs are displayed after uploading, and can also be viewed with the command wrangler mtls-certificate list.

Adding an mTLS certificate binding includes a variable in the Worker’s environment on which the fetch() method is available. This fetch() method uses the standard Fetch API and has the exact same signature as the global fetch, but always presents the client certificate when establishing the TLS connection.

Note

mTLS certificate bindings present an API similar to service bindings.

Interface

JavaScript

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

TypeScript

interface Env {
  MY_CERT: Fetcher;
}

export default {
    async fetch(request, environment): Promise<Response> {
        return await environment.MY_CERT.fetch("https://a-secured-origin.com")
    }
} satisfies ExportedHandler<Env>;