Cross-site scripting (XSS) attacks are a common vulnerability in web applications, and one of the most dangerous types is DOM-based XSS. This form of XSS occurs when the Document Object Model (DOM) of a web page is manipulated to execute malicious scripts. In this blog, we'll explore DOM-based XSS, how it works, and how you can protect your applications from these attacks with real-world example code.

What is DOM-based XSS?

DOM-based XSS is a type of XSS attack where the vulnerability lies in the client-side code rather than the server-side code. It occurs when a web application uses data from an untrusted source, such as user input, and writes it to the DOM without proper validation or escaping. This can lead to the execution of malicious scripts within the context of the web page, allowing attackers to steal data, hijack sessions, and more.

How DOM-based XSS Works

Let's break down a simple scenario to understand how an attacker could exploit DOM-based XSS:

Vulnerable Web Application Example
Consider a simple web page that displays a greeting message using user input from the URL hash.

How an Attacker Exploits the Vulnerability

1. Crafting a Malicious URL: An attacker can craft a URL that includes malicious JavaScript code in the URL hash. For example:

https://xyz.com/#

2. Sharing the Malicious URL: The attacker shares this URL with potential victims, who might click on it without suspicion. The attacker can distribute this link via email, social media, or any other means.

3. Exploiting the Vulnerability: When a victim visits the malicious URL, the web application extracts the value from the URL hash and inserts it into the DOM. The malicious script executes in the context of the web page.

Result: The victim sees an alert box with the message 'XSS', indicating that the script has executed. In a real attack, the malicious script could perform actions like stealing cookies, capturing keystrokes, or redirecting the user to a phishing site.

    var userInput = window.location.hash.substring(1);
    document.getElementById('message').innerHTML = "Hello, "   userInput   "!";
    // This results in: Hello, !
    // The alert will pop up

Preventing DOM-based XSS

To protect against DOM-based XSS, follow these best practices:

1. Sanitize and Escape User Input: Always sanitize and escape any user input before inserting it into the DOM. Use libraries like DOMPurify to sanitize HTML.

2. Use Safe DOM Manipulation Methods: Instead of using innerHTML, use safer methods like textContent or createElement and appendChild.

3. Content Security Policy (CSP): Implement a strong CSP to restrict the sources from which scripts can be loaded and executed.

DOM-based XSS is a critical security risk that can compromise your web application and user data. By following best practices such as sanitizing and escaping user input, using safe DOM manipulation methods, and implementing a robust Content Security Policy, you can significantly reduce the risk of DOM-based XSS attacks.

Stay vigilant and ensure your JavaScript applications are secure from these and other vulnerabilities. If you have any questions or need further assistance, feel free to reach out in the comments below.