$500 OTP Bypass: The Duplicate That Taught a Bigger Lesson

Disclaimer: This post is for educational and defensive learning only.
It explores a real-world bug bounty finding reported by another security researcher.
The purpose is to help developers, testers, and learners understand how logic flaws in OTP verification can compromise authentication workflows - and how to prevent them.

🧩 Introduction: The Beauty of Broken Logic

Every bug bounty hunter dreams of that one alert - the validation email that says “Rewarded: $500.”
But behind every payout, there are countless duplicates, dead ends, and near-misses that shape the hunter’s instincts.

This story isn’t about the bounty. It’s about the lesson behind a duplicate, and how a researcher’s curiosity turned a routine OTP verification flow into a critical bypass discovery.

It’s the perfect case study of why authentication logic testing still matters - and how skipping a single validation check can expose an entire user flow.

🧪 The Setup: A Normal OTP Flow That Hid a Surprise

The target was a typical web application with a standard signup/login workflow.
New users had to verify their email addresses using a 6-digit One-Time Password (OTP) sent via email before accessing their dashboard.

On paper, this was a sound implementation:

1. User registers an account.
2. System sends an OTP to their email.
3. User enters the OTP → verification successful.
4. Access is granted to internal routes like /dashboard, /settings, or /profile/edit.

The researcher followed this flow, completed verification, and began exploring the internal endpoints.

🧭 Endpoints of interest were:

• /dashboard
• /settings
• /profile/edit

At this point, everything behaved normally. Verification worked, and access was granted post-OTP confirmation.

But the story doesn’t end there.

🔁 The Second Attempt: Curiosity Pays Off

Like any good security researcher, the hunter decided to test the process again, but this time with a twist.

They created a second account, not to bypass anything - but to verify consistency across new sessions and users.

The app once again sent a 6-digit OTP email.
However, instead of entering the code, the researcher had a thought:

“What if I just skip the OTP screen and try accessing a known internal path directly?”

They copied /profile/edit - a path they knew existed - and pasted it straight into the browser’s address bar.

Then they hit Enter.

💥 Access granted.

No OTP.
No validation.
No warning.

They were inside the new account’s internal panel - completely unverified.

This was not a visual glitch or client-side trick. It was a full backend logic flaw that allowed users to bypass email/OTP verification entirely.

🧠 Why This Was Serious: A Logic Flaw with Real Impact

This wasn’t just a minor misconfiguration - it was a fundamental trust failure in the authentication layer.

🔓 What Happened Technically

The web application likely used session-based access control without enforcing verification status server-side.

When the user registered, the system probably:

• Created a user session immediately after registration.
• Flagged a “verified” status but didn’t validate it on every route.
• Failed to enforce OTP verification before granting access to protected pages.

In simpler terms:

The system created a door that was supposed to be locked - but forgot to check the key when someone walked in.

💣 The Potential Impact

Such a flaw could have serious implications:

• Automated account creation without email ownership.
• Spam accounts or bot registrations overwhelming the system.
• Bypass of 2FA/identity verification, allowing attackers to interact with core features.
• Data exposure or unauthorized access if combined with other insecure endpoints.

Even if it didn’t immediately grant administrative control, this bug undermined the trust boundary that email verification establishes - a critical piece of most modern signup systems.

📤 Reporting the Bug: The $500 That Got Away

The researcher crafted a clear, well-documented report including:

• Step-by-step reproduction instructions.
• Screenshots of the OTP flow and bypass.
• Technical analysis of potential backend validation issues.
• Recommended fixes for enforcing verification checks.

They submitted the report to the company’s bug bounty program, expecting a positive outcome.

Within a few days, the triager responded:

✅ “Valid issue.”
✅ “High impact confirmed.”

Then came the inevitable:

❗“Duplicate of a previously reported vulnerability.”

And with that, the payout - a $500 reward - slipped away.

💸 The “Almost” Payout: When Duplicates Teach the Best Lessons

Every bug hunter knows the bittersweet sting of duplicates.
You find the bug, prove the impact, and then… someone else beat you by a few days.

But duplicates are not failures - they’re validation.
They prove that your thought process, methodology, and instincts are spot on.

In this case, the researcher learned the right way to think:

• Always test authentication boundaries.
• Always revisit known paths after flow disruptions.
• Always assume that “client-side restrictions” can be bypassed.

This logic flaw was simple - yet devastating - and that’s what made it beautiful.

🕵️‍♂️ Step-by-Step Breakdown: How It Could Be Found (Ethically)

If you’re learning bug hunting, here’s a high-level safe guide to understanding and testing similar issues - responsibly.

1. Map Every Auth Flow

   • Identify endpoints triggered during signup, login, and OTP verification.
   • Record internal paths post-verification (e.g., /dashboard, /user/settings).

2. Use a Second Account

   • Register a new user but don’t complete OTP verification.
   • Observe which cookies or tokens are set before OTP success.

3. Replay Known Paths

   • Try accessing known authenticated routes directly from the unverified account.
   • Watch server responses - 200 means accessible, 401/403 means blocked correctly.

4. Analyze Server Logic

   • Check whether the backend checks for verification flags (is_verified == true).
   • If not, you’ve found a broken access control flaw.

5. Document Everything

   • Use screenshots, session details, and clear explanations.
   • Always report ethically - never exploit real user data.

🧰 Developer’s Defensive Perspective

Developers can easily prevent such issues with proper server-side validation and middleware logic.

Here’s how:

# Pseudocode for enforcing OTP verification before access
def require_verification(user):
    if not user.is_verified:
        return redirect('/verify-otp')
    else:
        return proceed_to_dashboard()

Key Best Practices:

• Enforce verification flags server-side on all protected endpoints.
• Avoid granting session tokens before verification success.
• Apply rate-limiting and anti-automation for registration and OTP endpoints.
• Log and monitor unusual access to “restricted” routes.
• Use API middleware to standardize authorization checks.

🧭 Lessons for Hunters & Developers

For Bug Hunters:

• Always test multi-step flows twice - before and after critical checkpoints.
• Don’t ignore simple ideas - skipping one step can reveal deep logic flaws.
• Duplicates are milestones, not mistakes. They show you’re thinking like the best.
• Focus on reliability and reproducibility in reports - that’s how you earn respect.

For Developers:

• Treat every verification process as a security boundary.
• Centralize your access control logic - don’t rely on client indicators.
• Audit every “post-registration” route to confirm verification enforcement.

💬 Final Thoughts

This $500 OTP bypass wasn’t about the bounty - it was about perspective.

Security isn’t just about clever payloads or high-tech exploits.
Sometimes, it’s about noticing what’s missing - a simple check, a missing flag, a forgotten gate.

The researcher didn’t win money that day, but they walked away with something far more valuable:

A deeper understanding of how logic flaws are born - and how to spot them early.

So next time you hit a duplicate, don’t get discouraged.
Celebrate it. It means you’re thinking like a pro.

Because every “almost bounty” is just one bug closer to your breakthrough.

References

• OWASP Authentication Cheat Sheet
• PortSwigger: Logic Flaws in Authentication
• HackerOne Reports: OTP and Email Verification Bypasses
• Bugcrowd University: Authentication and Authorization Testing

Published on herish.me - helping the next generation of ethical hackers learn from real-world vulnerabilities.