Exposing a DoS Vulnerability in 43.5% of the Web

Denial of Service (DoS) attacks aim to disrupt the availability of a website or service by overwhelming it with a flood of requests. The attack’s primary goal is to exhaust the server’s resources—such as CPU, memory, or bandwidth—making it unable to handle legitimate traffic. In severe cases, this can crash the website, causing downtime and affecting business operations.

There are two main types of DoS attacks:

1. Volumetric Attacks: Focus on overwhelming the network with high traffic.
2. Application Layer Attacks: Target specific functionalities of an application to exhaust the server’s resources.

The vulnerability we will explore today is an Application Layer DoS attack targeting WordPress sites through the load-scripts.php file.

WordPress Overview

WordPress is the most widely used Content Management System (CMS), powering 43.5% of websites globally. This platform’s popularity stems from its flexibility, open-source nature, and extensive plugin ecosystem. However, with its large market share comes the responsibility of dealing with various security challenges, including the one we’re investigating today: a DoS vulnerability found in load-scripts.php, which potentially affects the majority of WordPress installations.

1. Understanding load-scripts.php

The load-scripts.php file is a core component of WordPress designed to enhance performance by concatenating multiple JavaScript files into a single request. This is primarily done to reduce the number of HTTP requests and improve loading speed, especially on the WordPress admin dashboard and login pages.

How load-scripts.php Works

1. Parameter Handling:

• It takes the load[] parameter, which specifies an array of JavaScript handles that need to be loaded.
• The handles are processed and concatenated into a single response.

2. Code Functionality:

[php]
$load = $_GET['load'];
if (is_array($load)) {
ksort($load);
$load = implode('', $load);
}

$load = preg_replace('/[^a-z0-9,_-]+/i', '', $load);
$load = array_unique(explode(',', $load));

if (empty($load)) {
header("$protocol 400 Bad Request");
exit;
}
[/php]

• The code sanitizes the input to remove any invalid characters, sorts the handles, and ensures uniqueness.
• It then retrieves the JavaScript files associated with these handles from the server’s file system, concatenating them into a single response.

3. Performance Benefits:

• By reducing the number of HTTP requests, load-scripts.php aims to improve page load speed and reduce server load under normal circumstances.

4. Security Weakness:

• The script is accessible to unauthenticated users, particularly on the wp-login.php page, making it vulnerable to abuse.

2. Analyzing the Vulnerability

Nature of the Vulnerability

The vulnerability in load-scripts.php allows attackers to launch a DoS attack by abusing the script’s functionality to concatenate and deliver a large number of JavaScript files in a single request.

• Unauthenticated Access: The file is accessible without authentication, meaning anyone—including attackers—can make requests to it.
• Massive Resource Consumption: Attackers can exploit the load[] parameter by including up to 181 valid JavaScript handles in a single request, forcing the server to load, concatenate, and deliver a large file.
• Repetition: When repeated rapidly, this request causes the server’s CPU, memory, and I/O to spike, resulting in a denial of service where legitimate users can no longer access the site.

Exploit in Action

An example of the attack URL:

[sourcecode language=”plain”]
https://WPServer/wp-admin/load-scripts.php?c=1&load[]=eutil,common,wp-a11y,…&ver=6.6.2
[/sourcecode]

In this URL, up to 181 script handles can be requested at once, overwhelming the server.

3. Our Lab Setup for Testing

To validate and explore this vulnerability, we created a controlled testing environment:

Tools Used

1. JavaScript-based HTML Tool:

• We developed a custom HTML page with JavaScript to automate the sending of high-frequency GET requests targeting load-scripts.php.
• The HTML page featured:
  • Start/Stop buttons to manage requests.
  • Real-time response tracking and error logging.

2. Local CORS Proxy Setup:

• We used the local-cors-proxy module to bypass CORS restrictions for testing.
• Command example:

[sourcecode language=”bash”]
npm install -g local-cors-proxy
lcp –proxyUrl https://domain.com
[/sourcecode]

3. Testing on Shared Hosting:

• We tested the vulnerability on a simulated shared hosting environment, mimicking average resource limitations.

4. Results & Analysis

Mathematical Breakdown of Resource Consumption

• Response Size per Request: 779,307 bytes (~0.77 MB)
• Total Data Transferred: 37.56 MB (as seen in the cPanel logs)
• Number of Requests Needed: Using the formula:
  TotalRequests = Math.ceil(Total Data Transferred / Data per Request) = Math.ceil(37.56 MB / 0.77 MB) = 49 requests

Impact on Shared Hosting

Shared hosting environments typically offer limited CPU and memory resources. For instance:

• Memory: 512 MB to 1 GB
• CPU: Restricted to a few cores with limited processing power

During our tests, we found that:

• Server Slowdown: The server started to slow down significantly after around 30 requests.
• DoS Condition: After ~49 requests, the server became unresponsive, confirming a successful denial of service.

5. Mitigation Strategies

Here are several ways to mitigate this vulnerability:

1. 1. Restrict Access:
      • Limit access to load-scripts.php to authenticated users only. This can be configured using .htaccess or WordPress security plugins.

[php]
<FilesMatch "load-(scripts|styles)\.php$">
Order Deny,Allow
Deny from all
Allow from 192.168.1.0/24 # Allow specific IP ranges, adjust as needed
</FilesMatch>
[/php]

[php]
add_action('init', function() {
if (is_admin() && isset($_SERVER['REQUEST_URI']) && strpos($_SERVER['REQUEST_URI'], 'load-scripts.php') !== false) {
if (!is_user_logged_in()) {
wp_die('Access denied. Please log in to view this page.', 'Unauthorized Access', 403);
}
}
});
[/php]

1. Rate Limiting:
   • Implement rate limiting using tools like Fail2Ban, Wordfence, or Cloudflare’s WAF to limit the number of requests that can be made to sensitive scripts.
2. Caching Solutions:
   • Use server-side caching tools like Memcached or Redis to reduce the load from repeated requests.
3. Cloud-based Security:
   • Use services like Cloudflare or Sucuri to filter out malicious requests before they reach the server.

Conclusion

This investigation demonstrates that 43.5% of websites running on WordPress are potentially vulnerable to a DoS attack via the load-scripts.php file. Despite the performance benefits it offers, its unauthenticated access presents a critical security risk. Proper mitigation measures—like rate limiting, authentication requirements, and cloud-based filtering—can help reduce the risk of exploitation.

With these strategies implemented, you can protect your WordPress site from potential application-layer DoS attacks, ensuring stable performance and availability for legitimate users.