CF-Incidents Workers - New RFC 9440 mTLS certificate fields in Workers

[ENF News Bot]

Four new fields are now available on request.cf.tlsClientAuth in Workers for requests that include a mutual TLS (mTLS) client certificate. These fields encode the client certificate and its intermediate chain in RFC 9440 ↗︎ format — the same standard format used by the Client-Cert and Client-Cert-Chain HTTP headers — so your Worker can forward them directly to your origin without any custom parsing or encoding logic.

New fields​

https://developers.cloudflare.com/changelog/rss/#new-fields

Field	Type	Description
certRFC9440	String	The client leaf certificate in RFC 9440 format (:base64-DER:). Empty if no client certificate was presented.
certRFC9440TooLarge	Boolean	true if the leaf certificate exceeded 10 KB and was omitted from certRFC9440.
certChainRFC9440	String	The intermediate certificate chain in RFC 9440 format as a comma-separated list. Empty if no intermediates were sent or if the chain exceeded 16 KB.
certChainRFC9440TooLarge	Boolean	true if the intermediate chain exceeded 16 KB and was omitted from certChainRFC9440.

Example: forwarding client certificate headers to your origin​

https://developers.cloudflare.com/c...ing-client-certificate-headers-to-your-origin

export default {
  async fetch(request) {
    const tls = request.cf.tlsClientAuth;

    // Only forward if cert was verified and chain is complete
    if (!tls || !tls.certVerified || tls.certRevoked || tls.certChainRFC9440TooLarge) {
      return new Response("Unauthorized", { status: 401 });
    }

    const headers = new Headers(request.headers);
    headers.set("Client-Cert", tls.certRFC9440);
    headers.set("Client-Cert-Chain", tls.certChainRFC9440);

    return fetch(new Request(request, { headers }));
  },
};

For more information, refer to Client certificate variables and Mutual TLS authentication.

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