JavaScript detections

  3 min read

JavaScript detections are another method that help Cloudflare identify bot requests.

These detections are optional, but lead to greater overall accuracy.

​​ What are JavaScript detections?

These detections are implemented via a lightweight, invisible JavaScript code snippet that follows Cloudflare’s privacy standards Open external link . JavaScript is injected only in response to requests for HTML pages or page views, excluding AJAX calls. API and mobile app traffic is unaffected. Additionally, code is not injected again until the current session expires. After page load, the script is deferred and utilizes a separate thread (where available) to ensure that performance impact is minimal.

The snippets of JavaScript will contain a source pointing to the challenge platform, with paths that start with /cdn-cgi/challenge-platform/...

​​ Implement JavaScript detections

To enable JavaScript Detections:

1. Log in to your Cloudflare dashboard Open external link and select your account and domain.
2. Go to Security > Bots.
3. Select Configure Bot Management.
4. For JavaScript Detections, switch the toggle to On.

Once you enable JavaScript detections, you can use the cf.bot_management.js_detection.passed field in Firewall rules (or the request.cf.botManagement.js_detection.passed variable in Workers).

When adding this field to Firewall rules, use it:

• On endpoints expecting browser traffic (avoiding native mobile applications or websocket endpoints).
• After a user’s first request to your application (Cloudflare needs at least one HTML request before injecting JavaScript detection).
• With the Managed Challenge action, because there are legitimate reasons a user might not have passed a JavaScript detection challenge (network issues, ad blockers, disabled JavaScript in browser, native mobile apps).

​​ Prerequisites

• You must have JavaScript detections enabled on your zone.
• You must have updated your Content Security Policy headers for JavaScript detections.
• You must not run this field on websocket endpoints.
• You must use the field in a custom rules expression that expects only browser traffic.
• The action should always be a managed challenge in case a legitimate user has not received the challenge for network or browser reasons.
• The path specified in the rule builder should never be the first HTML page a user visits when browsing your site.

cf.bot_management.js_detection.passed is used to indicate that a request has a Bot Management cookie present with a JavaScript detection value indicating it submitted the JavaScript detection test, and received a likely human scoring result.

The cf.bot_management.js_detection.passed field should never be used in a Firewall field that can run a user’s first request to a site. It is necessary to have at least one HTML request before Cloudflare can inject JavaScript detection.

Example with Workers
"botManagement": {
"jsDetection":

{ "passed": false }
,
},

​​ Content Security Policies (CSPs)

If you have a Content Security Policy (CSP), you need to take additional steps to implement JavaScript detections:

• Ensure that anything under /cdn-cgi/challenge-platform/ is allowed. Your CSP should allow scripts served from your origin domain (script-src self).
• If your CSP uses a nonce for script tags, Cloudflare will add these nonces to the scripts it injects by parsing your CSP response header.
• If your CSP does not use nonce for script tags and JavaScript Detection is enabled, you may see a console error such as Refused to execute inline script because it violates the following Content Security Policy directive: "script-src 'self'". Either the 'unsafe-inline' keyword, a hash ('sha256-b123b8a70+4jEj+d6gWI9U6IilUJIrlnRJbRR/uQl2Jc='), or a nonce ('nonce-...') is required to enable inline execution. We highly discourage the use of unsafe-inline and instead recommend the use CSP nonces in script tags which we parse and support in our CDN.