XSS Prevention Guide

TL;DR

This guide shows how to prevent Cross-Site Scripting (XSS) attacks by properly sanitising user input and escaping output. XSS allows attackers to inject malicious scripts into your website, potentially stealing data or hijacking accounts.

Understanding the Problem

XSS happens when a web application takes untrusted data from a user and includes it in its response without proper validation or encoding. This can allow an attacker to execute JavaScript code in another user’s browser.

Solution Steps

1. Identify Input Points: Find all places where your website accepts input from users. This includes:
   • Form fields (text boxes, dropdowns, etc.)
   • URL parameters (e.g., example.com?search=keyword)
   • Cookies
   • Any data source controlled by the user
2. Sanitise Input: Remove or encode potentially harmful characters from user input *before* storing it in your database or using it. The best approach depends on the type of input and how you’ll use it.
   • HTML Encoding: Convert special characters to their HTML entities (e.g., < for <, > for >, & for &). This prevents the browser from interpreting them as HTML tags.

     // Example in PHP
     $safe_input = htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');
     

   • JavaScript Encoding: Encode characters that have special meaning in JavaScript (e.g., quotes, backslashes). Use this when inserting data into JavaScript code.

     // Example in PHP
     $safe_input = json_encode($user_input);
     

   • URL Encoding: Encode characters that have special meaning in URLs. Use this when including data in URL parameters.

     // Example in PHP
     $safe_input = urlencode($user_input);
     

3. Escape Output: Encode user-supplied data *before* displaying it on the page. This is crucial, even if you’ve already sanitised the input.
   • Context Matters: The correct escaping method depends on where the data is being inserted (HTML context, JavaScript context, CSS context, URL context).
   • Use a Templating Engine: Many templating engines (e.g., Twig, Jinja2) provide automatic escaping features. Enable these features to help prevent XSS.

     // Example in Twig
     {{ user_input | escape('html') }}
     

4. Content Security Policy (CSP): Implement a CSP header to control the resources that your browser is allowed to load. This can help mitigate XSS attacks even if other protections fail.

   // Example HTTP Header:
   Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-cdn.example.com;
   

5. Regularly Update Libraries: Keep your web application frameworks and libraries up to date. Security vulnerabilities are often patched in newer versions.
6. Input Validation: While not a replacement for sanitisation/escaping, validate input to ensure it conforms to expected formats (e.g., email address, phone number). This can reduce the attack surface.

Important Considerations

• Never trust user input: Always assume that user input is malicious.
• Double-encode if necessary: In some cases, you may need to encode data multiple times.
• Test thoroughly: Test your application with various XSS payloads to ensure your protections are effective.