Configure JavaScript Challenge

Published April 5, 2023 | Last modified November 10, 2026

Objective

This guide provides instructions on how to configure the JavaScript challenge for the application traffic. The JavaScript challenge functionality enables the load balancer to perform client browser compatibility test by redirecting to a page with JavaScript. For more information on the load balancer, see Load Balancing and Service Mesh.

Enabling this functionality allows only those clients that are capable of executing JavaScript (mostly browsers) to complete the HTTP request.

When load balancer is configured to execute JavaScript challenge, it redirects the browser to an HTML page embedded with JavaScript for every new HTTP request. The Load balancer chooses a set of random numbers for every new client and sends these numbers along with an encrypted answer with the request such that it embed these numbers as input in the JavaScript.

JavaScript runs on the browser of requestor and performs a complex mathematical operation. The script submits the answer to the load balancer. Load balancer validates the answer by comparing the calculated answer with the decrypted answer (which was encrypted when it was sent back as a reply) and allows the request to the upstream server only if the answer is correct.

The load balancer tags response header with a cookie to avoid JavaScript challenge for subsequent requests.

The JavaScript challenge serves following purposes:

Validate that the request is coming from a browser that is capable of running JavaScript
Force the browser to run a complex operation that requires it to spend many CPU cycles. This slows down a potential DoS attack by making it difficult to launch a large request flood without having to spend even larger CPU cost at their end.

Prerequisites

The following prerequisites apply:

An F5 Distributed Cloud Account. If you do not have an account, see Getting Started with Console.
An HTTP load balancer advertising your application. See HTTP Load Balancer for instructions on configuring HTTP load balancer. See Deploy for deploying your applications in a Distributed Cloud network cloud or edge cloud.

Configuration

Enabling the JavaScript challenge requires you to optionally prepare a custom message page encoded in Base64 format and apply it in the HTTP load balancer configuration. This example shows preparing a custom page and enabling the JavaScript challenge with it.

Configuration Sequence

Enabling JavaScript challenge requires you to perform the following sequence of actions.

Phase	Description
Prepare Custom Page for Redirection.	Prepare a page to which the load balancer redirects.
Enable JavaScript Challenge	Apply the custom page in load balancer configuration.

Prepare Custom Page for Redirection

The load balancer redirects to a page with default message or a custom message while performing the JavaScript challenge. The custom message can be in plain text or an HTML element such as a paragraph and applied to the load balancer through its configuration option. This example shows preparing a custom message in HTML paragraph.

Perform the following to prepare a custom message.

Step 1: Create a file with your custom message.

Create a file with your message in plain text or embedded in an HTML element such as paragraph. This example shows a sample message in a paragraph element stored in the test-redirect file.

<p>

Hello! Please wait.

</p>

Step 2: Apply Base64 encoding to the file.

Convert the file using Base64 encoding and store the output string for later use.

openssl base64 -in test-redirect

PHA+CgpIZWxsbyAhISEgUGxlYXNlIHdhaXQgCgo8L3A+Cg==

Enable JavaScript Challenge

Step 1: Navigate to the HTTP Load Balancers page.

Select the Multi-Cloud App Connect service.
Select the desired namespace from the Namespace drop-down menu.
Navigate to Manage > Load Balancers > HTTP Load Balancers.

Figure: Load Balancers

Step 2: Enable security options for JavaScript challenge.

Click ... > Manage Configuration for your load balancer to open its configuration viewer.
Click Edit Configuration in the upper right.
In the Common Security Controls section, perform the following:
- Enable the Show Advanced Fields option.
- From the Malicious User Mitigation And Challenges menu, select Javascript Challenge.
- Select View Configuration.
- Optionally, configure the Javascript Delay and Cookie Expiration Period options.
- Under Custom Message for Javascript Challenge, select Base64.
- Paste the custom message into the box.
- Click Apply.
- Click Save and Exit.

Verify JavaScript Challenge Functionality

After enabling JavaScript challenge functionality, you can send request to the application domain from a browser and check if the load balancer redirects to the page you configured.

Step 1: Send request from the browser to your application domain.

Send a request by loading the domain specified in the load balancer configuration.

Step 2: Verify the JavaScript challenge.

Check if the custom page you configured is displayed. After the configured delay, verify if the load balancer redirects to your application site. This example shows that after the delay, the load balancer loads the requested hipster application page.

Figure: JavaScript Challenge Redirection Message Page

Note: You can check the status of the JavaScript challenge functionality for any issues. Go to the HTTP Load Balancers page and use the ... > Show Child Objects option of the load balancer. Then in the Route section, click View for your load balancer and use the drop-down menu to show route status for a particular site.