Cross-Site Scripting (XSS) is a web security vulnerability that allows attackers to inject malicious scripts into content that other users will view. This can lead to various attacks, such as stealing session cookies, defacing websites, or redirecting users to malicious sites.

How XSS Works

XSS occurs when an application includes untrusted data in a web page without proper validation or escaping. Attackers can exploit this by injecting scripts that execute in the context of the victim’s browser.

Types of XSS

1. Stored XSS (Persistent): The malicious script is stored on the server (e.g., in a database) and delivered to users whenever they access the affected page.
   • Example: A comment section where users can post comments without sanitization.

     • Payload:

       <script>alert('Hacked!');</script>
       
       

     • When a user submits this comment, it gets stored in the database. When other users view the comment, the script executes in their browsers, showing an alert box.

2. Reflected XSS (Non-Persistent): The attack is reflected off a web server, typically via URL parameters. The script is not stored; it is included in the response based on the request.
   • Example: A search page that includes user input directly in the response.

     • URL:

       <http://example.com/search?q=><script>alert('Hacked!');</script>
       
       

     • When the server processes this request and reflects it back without sanitization, the script executes immediately in the user's browser.

3. DOM-based XSS: The vulnerability exists in the client-side JavaScript. The attack manipulates the Document Object Model (DOM) to execute malicious scripts.
   • Example: A webpage that takes URL parameters and modifies the page content.

     • JavaScript:

       const userInput = window.location.hash.substring(1);
       document.getElementById('output').innerHTML = userInput;
       
       

     • If the URL is http://example.com/#<script>alert('Hacked!');</script>, the injected script executes when the page loads.

Consequences of XSS

• Session Hijacking: Attackers can steal session cookies, allowing them to impersonate users.
• Defacement: Malicious scripts can alter the appearance of a webpage.
• Phishing: Users can be redirected to fraudulent sites to steal credentials.

Prevention Techniques

1. Input Validation: Validate and sanitize all user inputs to ensure they conform to expected formats. Use libraries or frameworks that handle this automatically.
2. Output Encoding: Encode data before including it in HTML, JavaScript, or URL contexts to prevent it from being executed.
   • For example, convert < to <, > to >, etc.
3. Content Security Policy (CSP): Implement CSP headers to restrict sources of executable scripts. This can help prevent the execution of unauthorized scripts.
4. Use Safe APIs: Avoid using dangerous functions like innerHTML. Instead, use safer alternatives like textContent for adding text content to elements.
5. Regular Security Testing: Conduct security assessments, including automated scanning for XSS vulnerabilities.