Fix URL Verification Issues

TL;DR

URL verification problems often happen when a website isn’t correctly checking where links are pointing. This can let attackers redirect users to harmful sites or bypass security checks. We’ll cover common causes and how to fix them, focusing on server-side validation.

Understanding the Problem

URL verification is crucial for security. It ensures that when a user clicks a link, they go to the expected destination. If this process fails, attackers can exploit it through techniques like:

• Open Redirects: A website redirects users to an attacker-controlled site using a legitimate URL parameter.
• Bypassing Filters: Security filters are tricked into allowing malicious URLs.

Fixing URL Verification Issues

1. Server-Side Validation is Key
   • Never trust user input directly. Always validate any URL provided by a user on the server before using it. Client-side validation can be bypassed easily.
   • Use a whitelist approach: Define allowed domains or URL patterns and reject anything outside of that list. This is much safer than trying to blacklist malicious URLs (which are constantly changing).
2. Implement Strict Domain Checking

   Ensure the domain part of the URL matches your expected values.

3. Normalise URLs
   • Remove unnecessary characters: Strip out any extra spaces, tabs, or special characters from the URL.
   • Decode URL-encoded values: Use urldecode() to decode any encoded parts of the URL before validation. This prevents attackers from using encoding tricks to bypass filters.
4. Check for Redirect Chains

   Attackers might use multiple redirects to hide the final malicious destination. Limit the number of allowed redirects.

   = $max_redirects) {
     // Too many redirects - potential attack.
     echo "Too many redirects";
   }
   ?>

5. Use a Cybersecurity Library

   Consider using established cybersecurity libraries or frameworks that provide built-in URL validation and sanitisation functions. These are often well-tested and can help prevent common vulnerabilities.

6. Regularly Review Your Code

   Periodically review your code for potential URL verification issues, especially after making changes to redirect functionality or input handling.