[Turnstile] Tutorial for conditionally enforcing Turnstile with HTMLRewriter (#21311)

* [Turnstile] Tutorial for conditionally enforcing Turnstile with HTMLRewriter

* Apply suggestions from code review

Co-authored-by: Patricia Santa Ana <[email protected]>

* demo

* Update src/content/docs/turnstile/tutorials/conditionally-enforcing-turnstile.mdx

---------

Co-authored-by: Patricia Santa Ana <[email protected]>

Author: KianNH

src/content/docs/turnstile/tutorials/conditionally-enforcing-turnstile.mdx

@@ -0,0 +1,108 @@
+---
+title: Conditionally enforce Turnstile
+pcx_content_type: tutorial
+updated: 2025-04-01
+difficulty: Intermediate
+languages:
+  - TypeScript
+tags:
+  - Node.js
+sidebar:
+  order: 6
+---
+
+This tutorial explains how to conditionally enforce Turnstile based on the incoming request, such as a pre-shared secret in a header or a specific IP address.
+
+## Overview
+
+You may have setups such as automation that cannot load or run the Turnstile challenge. Using [`HTMLRewriter`](/workers/runtime-apis/html-rewriter/), this tutorial will demonstrate how to conditionally handle the [client-side widget](/turnstile/get-started/client-side-rendering/) and [siteverify API](/turnstile/get-started/server-side-validation/) when specific criteria are met.
+
+:::note
+While this tutorial removes Turnstile client-side elements when specific criteria are met, you could instead conditionally insert them.
+:::
+
+:::caution
+It is critical to make sure you are validating tokens with the siteverify API when your criteria for enforcing Turnstile are not met.
+
+It is not sufficient to only remove the client-side widget from the page, as an attacker can forge the request to your API.
+:::
+
+## Implementation
+
+This tutorial will modify the existing [Turnstile demo](https://github.com/cloudflare/turnstile-demo-workers/blob/main/src/) to conditionally remove the existing `script` and widget container elements.
+
+```diff title="src/index.mjs" lang="js"
+export default {
+  async fetch(request) {
+    // ...
+
++    if (request.headers.get("x-bypass-turnstile") === "VerySecretValue") {
++      class RemoveHandler {
++        element(element) {
++          element.remove();
++        }
++      }
++
++      return new HTMLRewriter()
++        // Remove the script tag
++        .on(
++          'script[src="https://challenges.cloudflare.com/turnstile/v0/api.js"]',
++          new RemoveHandler(),
++        )
++       // Remove the container used in implicit rendering
++				.on(
++					'.cf-turnstile',
++					new RemoveHandler(),
++				)
++       // Remove the container used in explicit rendering
++				.on(
++					'#myWidget',
++					new RemoveHandler(),
++				)
++        .transform(body);
++    }
+
+    return new Response(body, {
+      headers: {
+        "Content-Type": "text/html",
+      },
+    });
+  },
+};
+```
+
+## Server-side integration
+
+We will exit early in our validation if the same logic we used to remove the client-side elements is present.
+
+:::caution
+The same logic must be used in both the client-side and the server-side implementations.
+:::
+
+```diff lang="js" title="src/index.mjs"
+async function handlePost(request) {
++  if (request.headers.get("x-bypass-turnstile") === "VerySecretValue") {
++    return new Response('Turnstile not enforced on this request')
++  }
+	// Proceed with validation as normal!
+	const body = await request.formData();
+  // Turnstile injects a token in "cf-turnstile-response".
+  const token = body.get('cf-turnstile-response');
+  const ip = request.headers.get('CF-Connecting-IP');
+  // ...
+}
+```
+
+With these changes, Turnstile will not be enforced on requests with the header `x-bypass-turnstile: VerySecretValue` present.
+
+## Demonstration
+
+After running `npm run dev` in the project folder, you can test the changes by running the following command:
+
+```sh
+curl -X POST http://localhost:8787/handler -H "x-bypass-turnstile: VerySecretValue"
+```
+
+```txt output
+Turnstile not enforced on this request
+```