How Curiosity Led Me to a Critical Authentication Bypass

Hello guys! 👋 It's been a long time, but I'm back with another interesting bug bounty story. Around two weeks ago, I found one of the most exciting vulnerabilities of my bug hunting journey, and today I want to share how curiosity led me to a Critical Authentication Bypass.

Like every new target, I started by exploring the application. My first step is always to understand every feature and functionality before testing anything. After spending some time exploring, I decided to focus on the login functionality.

I began with the usual tests:

SQL Injection

Pre-Account Takeover

Unfortunately, nothing worked. Instead of wasting time trying the same payloads repeatedly, I took a short break. About an hour later, I came back with a fresh mindset and started looking at the login flow again.

This time, I noticed that the application appeared to rely heavily on server-side validation. That observation made me curious, so I started experimenting with the request itself.

First, I removed the email field completely. ➡️ The server returned an error. Next, I sent the email field with a null value. ➡️ Another error. Then I removed the password field.

➡️ Surprisingly, the application returned a completely different error message. At that moment, I knew something wasn't behaving as expected.

Out of curiosity, I modified the request one more time.

Instead of sending the password parameter, I replaced it with a user_id parameter and submitted the request. I couldn't believe what happened next. I was successfully logged into my own account without providing the correct password.

To verify the impact, I tested it on another account that I owned for security testing purposes by supplying its email address and corresponding user ID.

Boom.

The application authenticated me into that account as well. At that point, it was clear that I had discovered a Critical Authentication Bypass that could allow unauthorized access to user accounts.

I immediately documented everything with a clear proof of concept and responsibly reported the vulnerability to the security team.

After about two weeks, I received their response:

"Good catch!"

The report was accepted as Critical, and I was rewarded with a bounty. 💰🎉

Lessons Learned This finding reminded me of something every bug hunter should remember: Don't stop after common payloads fail. Small differences in server responses can reveal major vulnerabilities. Curiosity often leads to the best discoveries.

Sometimes taking a short break gives you a completely new perspective. This bug wasn't found by using a fancy payload or an automated scanner. It was found simply by observing how the application behaved, asking "Why is this response different?", and following that curiosity until it revealed a critical vulnerability.

Happy Hunting! 🚀