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Introduction:
Reflected Cross-Site Scripting (XSS) remains one of the most prevalent and dangerous web application vulnerabilities, allowing attackers to execute malicious scripts in a victim’s browser. As demonstrated by the recent discovery on fatora.io, this flaw is a prime target in bug bounty programs due to its potential for cookie theft, session hijacking, and defacement. Understanding the mechanics of reflected XSS is crucial for both offensive security professionals seeking to identify it and developers tasked with eradicating it from their code.
Learning Objectives:
- Understand the fundamental mechanics of a reflected XSS attack and how to identify vulnerable parameters.
- Learn practical techniques for crafting and testing XSS payloads against modern web applications.
- Master the implementation of robust mitigation strategies, including Content Security Policy (CSP) and input sanitization.
You Should Know:
1. Identifying Reflected XSS Vectors
The first step is to probe every user-input point for reflection. This includes URL parameters, form fields, and HTTP headers.
Command/Tool: Basic cURL to test parameter reflection
curl -s "https://target.com/search?query=<test123>" | grep -n "test123"
Step-by-Step Guide:
This command sends a request to the `search` endpoint with a test string in the `query` parameter. The `-s` flag silences the progress meter, and the `grep -n` command searches the HTML response for the exact string “test123”, displaying the line number where it appears. If your input is reflected unchanged in the response, the parameter is a candidate for XSS. Replace `
2. Crafting Basic XSS Payloads
When simple script tags are blocked, alternative payloads are necessary.
Code Snippet: Classic and Obfuscated XSS Payloads
<!-- Classic Script Tag -->
<script>alert(document.domain)</script>
<!-- Event Handler in HTML Tag -->
<img src=x onerror=alert(1)>
<!-- Using JavaScript Protocol -->
<a href="javascript:alert('XSS')">Click</a>
Step-by-Step Guide:
Start with the basic `