URL Injection Attack

TL;DR

Someone is trying to inject malicious JavaScript code into your website via a URL. This usually happens when user input isn’t properly checked before being used in the URL. We’ll cover how to prevent this.

Understanding the Attack

Attackers try to insert harmful code, often JavaScript, into URLs that your website uses. If your site then displays parts of these URLs without cleaning them first, the attacker’s code can run in a visitor’s browser. This could steal cookies, redirect users, or deface your site.

How to Prevent URL Injection Attacks

1. Validate All User Input: The most important step! Never trust data directly from the user (forms, query parameters, etc.).
• Whitelisting is best: Only allow known good characters. For example, if you expect a name, only allow letters and spaces.
• Blacklisting is less secure: Trying to block specific bad characters (like <, >, ") can be bypassed easily.
2. Encode Output: Before displaying any user-provided data in a URL, encode it properly.
   • URL Encoding: Use the correct encoding for URLs (percent-encoding). This converts unsafe characters into a safe format.

     // Example using PHP
     $url = urlencode($userInput);
     echo "https://example.com?param=" . $url;
     

   • HTML Encoding: If the URL is displayed within HTML, also encode it for HTML to prevent XSS (Cross-Site Scripting) attacks.

     // Example using PHP
     $url = htmlspecialchars($userInput, ENT_QUOTES, 'UTF-8');
     echo "https://example.com?param=" . $url;
     

3. Use Parameterized Queries (if applicable): If you’re using a database, always use parameterized queries or prepared statements.

   This prevents attackers from injecting SQL code through the URL.

4. Content Security Policy (CSP): Implement a strong CSP to control which resources your website can load. This limits the damage an attacker can do even if they manage to inject some JavaScript.

   // Example of setting CSP header in .htaccess
   Header set Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted-cdn.com"

5. Regularly Scan Your Website: Use a web vulnerability scanner to identify potential weaknesses, including URL injection vulnerabilities.
6. Keep Software Updated: Ensure your website platform (WordPress, Drupal, etc.) and all plugins/libraries are up-to-date. Updates often include security fixes.

Example Scenario

Let’s say you have a search box that passes the query to a URL like this: https://example.com/search?q=[user_query].

If an attacker enters <script>alert('XSS')</script> as their query, without proper validation and encoding, the resulting URL becomes: https://example.com/search?q=<script>alert('XSS')</script>.

When this page loads, the browser will execute the JavaScript code, displaying an alert box. This is a simple example of how URL injection can lead to XSS attacks.