DOM XSS in jQuery anchor href attribute | شرح ثغرة XSS - Portswigger

Introduction

This tutorial focuses on discovering and mitigating DOM-based Cross-Site Scripting (XSS) vulnerabilities using the jQuery anchor href attribute. It is tailored for beginners interested in web application security, particularly in the context of bug bounty programs. By the end of this guide, you will understand how to identify these vulnerabilities, analyze affected code, and exploit them safely in a controlled environment.

Step 1: Understand jQuery

• jQuery is a fast, small, and feature-rich JavaScript library.
• It simplifies HTML document traversing, event handling, and animation.
• Developers often prefer jQuery for its ease of use and cross-browser compatibility.

Step 2: Learn about the Anchor href Attribute

• The href attribute in anchor (<a>) tags specifies the URL of the page the link goes to.
• It can be manipulated in ways that lead to security vulnerabilities, specifically when user input is not properly sanitized.

Step 3: Overview of the Lab Environment

• Familiarize yourself with the PortSwigger lab designed for practicing DOM XSS vulnerabilities.

• Requirements to access the lab:

  • A browser (preferably with developer tools enabled).
  • Understanding basic JavaScript and jQuery operations.

• Lab URL: PortSwigger Lab

Step 4: Analyze How the Vulnerability Occurs

• Vulnerabilities occur when user input is directly inserted into the DOM without validation.
• Common scenarios include:
  • Using user input to set the href attribute.
  • Failing to escape special characters, allowing script injection.

Step 5: Code Analysis

• Review the vulnerable code snippet. For example:

  $(document).ready(function() {
      var userInput = getUrlParameter('input'); // Assume this function retrieves a query parameter
      $('#link').attr('href', userInput);
  });
  

• In this code:
  • User input is taken directly and set as the href of an anchor tag.
  • If the input is something like javascript:alert(1), clicking the link will execute the alert.

Step 6: Inject JavaScript Code

• To exploit the vulnerability, you can craft a malicious URL.
• For example, you might append the following to the lab URL:

  ?input=javascript:alert('XSS')
  

• This will execute the alert when the link is clicked, demonstrating the vulnerability.

Conclusion

In this tutorial, you learned about DOM-based XSS vulnerabilities through the jQuery anchor href attribute. Understanding how jQuery works and how user input can lead to security issues is crucial for developing secure applications. To further your learning, consider exploring more advanced security topics and practicing in controlled environments like the PortSwigger labs. Always remember to report any vulnerabilities responsibly.