DOM XSS in document.write sink | شرح ثغرة XSS - Portswigger

Introduction

This tutorial provides a step-by-step guide on understanding and exploiting DOM-based Cross-Site Scripting (XSS) vulnerabilities using the document.write method. It is particularly useful for beginners in cybersecurity, especially those interested in web application security and bug bounty programs. We will cover the analysis of vulnerable code, the injection of payloads, and the differentiation between DOM XSS and reflected XSS.

Step 1: Access the PortSwigger Lab

• Visit the PortSwigger lab for DOM XSS:
  • URL: PortSwigger Lab
• Familiarize yourself with the lab environment, which is designed to practice identifying and exploiting XSS vulnerabilities.

Step 2: Start the Testing Process

• Begin by analyzing the web application provided in the lab.
• Look for areas in the code where user input is processed and rendered on the page using document.write.

Step 3: Analyze the Vulnerable Code

• Identify how data is manipulated in the application. Focus on:
  • The use of document.write for inserting content into the DOM.
  • Any user input that is directly written to the document without proper sanitization.
• Common indicators of vulnerability include:
  • Direct usage of user inputs without escaping.
  • Function calls that manipulate the DOM based on user data.

Step 4: Inject Payloads

• To exploit the identified vulnerability, inject malicious payloads. Here are the steps:
  • Use simple JavaScript payloads to test for XSS. For example:

    <script>alert('XSS');</script>
    

  • Submit the payload through any input field susceptible to XSS.
  • Monitor the output to see if the script executes, indicating a successful injection.

Step 5: Differentiate Between DOM XSS and Reflected XSS

• Understand the key differences:
  • DOM XSS occurs when the client-side script writes to the DOM directly without validating or encoding user inputs.
  • Reflected XSS happens when an attacker sends a malicious link that reflects back the payload from the server, often used in phishing attacks.
• Recognizing these differences is crucial for identifying the type of vulnerability you are dealing with.

Conclusion

In this tutorial, we explored how to identify and exploit DOM-based XSS vulnerabilities using the document.write method. Key steps included accessing the PortSwigger lab, analyzing vulnerable code, injecting payloads, and understanding the differences between DOM XSS and reflected XSS.

For further learning, practice injecting different payloads and explore additional resources on XSS vulnerabilities and prevention strategies. This knowledge is foundational for improving your skills in cybersecurity and enhancing web application security.