$9,500 Bug Bounty: How I Chained IDOR and BAC to Discover This Critical Vulnerability

Dive into my journey of discovering a complex vulnerability chain that led to a complete account takeover — and how you can improve your bug-hunting skills.

Photo by Markus Spiske on Unsplash

A detailed walkthrough of identifying, exploiting, and responsibly disclosing a critical security flaw


As a security researcher, every day brings new challenges and opportunities. But some discoveries stand out more than others. This is how I uncovered a critical vulnerability chain that started with a seemingly simple Insecure Direct Object Reference (IDOR) and escalated into something more significant.

The discovery process wasn’t straightforward. It required patience, persistence, and a deep understanding of how different security flaws can interact with each other. What began as a routine security assessment turned into one of my most interesting findings to date.

The target application was a financial services platform serving millions of users worldwide. With high-value transactions flowing through the system daily, the stakes couldn’t have been higher. The application used a modern tech stack, including:

// Main technology stack
const techStack = {
frontend: "React.js",
backend: "Node.js",
database: "MongoDB",
authentication: "JWT-based",
apis: "RESTful + GraphQL"
};

When examining the application’s structure, I noticed something interesting in how it handled user sessions and access controls. The authentication mechanism relied heavily on Bearer Authentication Controls (BAC), but there were subtle inconsistencies in how these controls were implemented across different endpoints.

The vulnerability chain I discovered consisted of multiple components working together. The initial IDOR vulnerability allowed me to access user profile data I shouldn’t have been able to see. This might seem relatively low-impact on its own, but it was just the beginning.

GET /api/v1/users/profile/12345 HTTP/1.1
Host: target-application.com
Authorization: Bearer [REDACTED]

What made this particularly interesting was how the IDOR interacted with the application’s BAC implementation. By manipulating certain request parameters, I could exploit this interaction to escalate my privileges significantly.

The exploitation process required careful attention to detail and systematic testing. Here’s how the vulnerability chain worked:

First, I identified the IDOR vulnerability by analyzing the application’s API endpoints. The key was in how the application handled user profile requests:

def get_user_profile(user_id):
# Vulnerable implementation
profile = db.query(f"SELECT * FROM users WHERE id = {user_id}")
return profile # No proper authorization check

The BAC bypass came next, leveraging the information gathered from the IDOR exploitation:

POST /api/v1/auth/elevate HTTP/1.1
Host: target-application.com
Content-Type: application/json

{
"user_id": "12345",
"session_token": "[REDACTED]",
"elevation_type": "admin"
}

At its core, this vulnerability chain existed because of several fundamental security oversights:

Improper Access Controls: The application failed to properly validate user permissions across all endpoints consistently.Broken Authentication Flow: The BAC implementation contained logic flaws that could be exploited when combined with other vulnerabilities.

The impact of this vulnerability chain was significant. An attacker could potentially gain unauthorized access to any user account, leading to serious privacy violations and potential financial losses.

This discovery reinforced several crucial security principles. Defense in depth isn’t just a buzzword — it’s essential for preventing vulnerability chains like this one. Proper implementation of access controls needs to happen at every layer of the application.

Here’s what developers can do to prevent similar vulnerabilities:

// Proper implementation example
const validateUserAccess = async (userId, requestingUser) => {
if (!requestingUser.hasPermission('VIEW_PROFILE')) {
throw new UnauthorizedError();
}

if (userId !== requestingUser.id && !requestingUser.isAdmin) {
throw new ForbiddenError();
}

return true;
}

Discovering this vulnerability chain was a reminder that security is often about seeing the bigger picture. Individual vulnerabilities might seem minor, but when chained together, they can create critical security risks.

The responsible disclosure process went smoothly, and the development team was quick to implement fixes. This experience highlights the importance of continued security research and the value of responsible bug bounty programs in improving overall internet security.

Remember: Security is not a destination but a journey. Keep learning, keep testing, and always approach applications with a creative mindset.