How a DoS Vulnerability in WordPress Poses Risks to over 40% of Websites Worldwide

Introduction

WordPress, the world’s leading Content Management System (CMS), powers over 40% of all websites globally. While its ease of use and flexibility make it a popular choice for businesses, bloggers, and developers, this extensive adoption also makes it an attractive target for cyberattacks. One such overlooked vulnerability is found in the core WordPress file load-scripts.php, which attackers can exploit to launch a Denial of Service (DoS) attack.

This article explores the nature of the vulnerability, why it’s a concern, and how to mitigate it.

What is a Denial of Service (DoS) Attack?

A Denial of Service (DoS) attack is a cyber assault where the attacker attempts to make a website or service unavailable to its intended users by overwhelming it with a flood of illegitimate requests. This sudden surge of requests drains the server’s resources, leading to slowdowns or complete shutdowns.

The DoS attack on load-scripts.php in WordPress is categorized as an application-layer DoS, where specific functions of a website are targeted to exhaust its resources rather than flooding the network.

Understanding load-scripts.php in WordPress

The load-scripts.php file is an essential part of WordPress, designed to boost performance. It reduces the number of HTTP requests by concatenating multiple JavaScript files into a single request, thereby improving loading speeds.

How load-scripts.php Works

1. Input Handling:The file accepts a parameter called load[], an array of JavaScript handles, and processes the handles for concatenation.

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

2. Sanitization:It cleans the input to remove invalid characters and duplicate values, ensuring that only valid JavaScript handles are processed.

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

3. Concatenation:The file retrieves the JavaScript files associated with the requested handles, concatenates them, and delivers the combined response to the client.

The Vulnerability in load-scripts.php

The vulnerability in load-scripts.php emerges from its unauthenticated accessibility, which allows anyone—including attackers—to make requests without logging in. By leveraging this feature, an attacker can cause the server to process a large number of JavaScript files in a single request, consuming significant server resources.

Key Characteristics of the Vulnerability

• Unauthenticated Access: The file does not require authentication, making it easily accessible to attackers.
• Massive Resource Consumption: Attackers can pass up to 181 valid handles in a single request, causing a massive CPU and memory load on the server.
• High Amplification Potential: Since the file processes and returns a large amount of data, attackers can achieve high amplification with minimal effort.

Exploiting load-scripts.php for DoS Attacks

The attacker crafts a malicious URL targeting the load-scripts.php file with an excessive number of script handles:

https://example.com/wp-admin/load-scripts.php?c=1&load[]=jquery,jquery-ui-core,...

The request forces the server to process and concatenate all specified scripts. When repeated rapidly, this technique overwhelms server resources, leading to a Denial of Service.

Setting Up a Testing Environment

To explore and validate this vulnerability, we built a controlled testing environment using local resources. The aim was to understand the attack dynamics without impacting any live site.

Tools and Technologies Used

• Local WordPress Installation: A WordPress setup on a local machine ensured no harm to production sites during testing.
• JavaScript-based Testing Tool: We developed an HTML interface with JavaScript to automate the sending of high-frequency requests to the load-scripts.php file.
• Local CORS Proxy: We used the local-cors-proxy package to bypass CORS restrictions and enable seamless testing.

  
  npm install -g local-cors-proxy
  lcp --proxyUrl https://example.com
      

• Simulated Shared Hosting: We configured limited resources to mimic the behavior of a shared hosting environment during an attack.

Results of the Attack Simulation

• Average Request Size: Each request generated an average response of 779,307 bytes (~0.77 MB).
• Total Data Transferred: Over 49 requests, approximately 37.56 MB of data was transferred.
• Server Behavior:
  • Initial Slowdown: The server started experiencing slowdowns after 30 requests.
  • Complete Denial of Service: The server became unresponsive after 49 requests, confirming that this vulnerability could easily be exploited for DoS attacks.

Mitigating the Vulnerability in load-scripts.php

To protect your WordPress site from this potential DoS attack, here are some effective strategies:

1. Restrict Access Using .htaccess

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

2. Enforce Authentication for the File

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);
        }
    }
});

3. Implement Rate Limiting

Rate limiting tools, such as Wordfence, Fail2Ban, or Cloudflare’s Web Application Firewall (WAF), can be used to restrict the number of requests to sensitive endpoints.

4. Server-side Caching Solutions

Using caching tools like Memcached or Redis can help reduce the server load by caching responses and minimizing repetitive processing.

Conclusion

The load-scripts.php vulnerability in WordPress is a critical concern, especially for websites using shared hosting environments. While the file is designed to optimize performance, its lack of authentication presents a significant security risk.

By implementing a combination of rate limiting, authentication checks, and access restrictions, you can effectively mitigate this risk and enhance your site’s resilience against potential DoS attacks.

As WordPress continues to be the backbone of the modern web, maintaining the security of its core components is crucial. Site owners, developers, and security professionals should be aware of vulnerabilities like these and take proactive measures to safeguard their websites.