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Configure mTLS

When you specify API hosts in mTLS authentication, Cloudflare will block all requests that do not have a certificate for mTLS authentication.

Prerequisites

Before you can protect your your API or web application with mTLS rules, you need to:

Create an mTLS rule

To create an mTLS rule in the Cloudflare dashboard, follow these steps:

  1. Log into your Cloudflare account and select your application.

  2. Go to Firewall > Firewall Rules.

  3. Click Create a mTLS rule.

  4. Enter the following information:

  5. By default, your rule will have a configuration similar to the following:

    ExpressionAction
    (http.host in {"api.example.com"} and not cf.tls_client_auth.cert_verified)Block

    To make this rule active, click Deploy. To add additional firewall logic — such as checking for revoked certificates — click Use firewall rule builder.

  6. Once you have deployed your mTLS rule, any requests without a Cloudflare-issued client certificate will be blocked.

Expression Builder

To review your mTLS rule in the Firewall Rules Expression Builder, click the wrench icon associated with your rule.

In the Expression Preview, your mTLS rule includes a compound expression formed from two simple expressions joined by the and operator.

The first expression uses the http.host field, combined with the in operator, to capture the hosts your mTLS rule applies to.

The second expression — not cf.tls_client_auth.cert_verified — returns true when a request to access your API or web application does not present a valid client certificate.

Because the action for your rule is Block, only requests that present a valid client certificate can access the specified hosts.

Check for revoked certificates

To check for revoked client certificates, you can either add a new mTLS rule or add a new expression to the default rule.

When a request includes a revoked certificate, the cf.tls_client_auth.cert_revoked field is set to true. If you combined this with the default mTLS rule, it would look similar to the following:

(http.host in {"api.theburritobot.com"}) and (not cf.tls_client_auth.cert_verified or cf.tls_client_auth.cert_revoked)