Hi everyone, I'm Omar Ahmed — it's been a while since my last post.

Whenever I open a new application, I follow a simple habit: before exploring features deeply, I open Burp Suite. I interact with the app normally, explore its flows, and then review the HTTP history carefully, looking for anything unusual or potentially risky.

This mindset has helped me identify multiple issues before and this case was a good reminder of how small oversights can lead to serious impact.

Understanding the Signup Flow

While analyzing the signup process, I noticed that the application required users to verify their accounts using an OTP (One-Time Password) sent during registration. This is a standard and necessary security control.

However, one detail immediately caught my attention:

  • The OTP was only 4 digits long

By itself, a short OTP is not automatically a vulnerability. The real question is always about what security controls surround it.

What Is a "No Rate Limiting" Vulnerability?

Rate limiting is a security mechanism that restricts how many requests a user or IP address can send to a specific endpoint within a given time.

A No Rate Limiting vulnerability occurs when:

  • An endpoint accepts unlimited requests
  • No delays, lockouts, or throttling exist
  • Attackers can automate requests without restriction

This is especially dangerous on:

  • Login endpoints
  • OTP verification endpoints
  • Password reset flows

Without rate limiting, attackers can brute-force credentials or OTPs efficiently.

The Critical Observation

After monitoring the OTP verification endpoint in Burp, it became clear that no effective rate limiting was implemented.

This instantly changed the risk level.

A 4-digit OTP means:

  • Only 10,000 possible combinations
  • With no rate limiting, brute forcing becomes practical
  • Without lockouts or delays, protection is minimal

At this point, the vulnerability was no longer theoretical.

Exploitation: When Small Issues Combine

By automating OTP attempts against the verification endpoint, I was able to submit thousands of guesses without restriction.

Eventually, a correct OTP was accepted — without ever receiving or reading the real OTP message.

The verification step was effectively bypassed.

What Is Account Takeover (ATO)?

Account Takeover (ATO) is a security vulnerability where an attacker gains unauthorized access to a user's account.

ATO can result from:

  • Authentication bypass
  • Weak or brute-forceable OTPs
  • Missing rate limiting
  • Poor session or verification controls

In this case:

  • OTP verification was bypassed
  • The account was accessed successfully
  • This led to a full Account Takeover

Impact and Security Lessons

This vulnerability did not require advanced exploitation techniques or complex logic. It happened because a critical defensive control was missing.

Key Takeaways:

  • OTP length alone does not guarantee security
  • Rate limiting is mandatory, not optional
  • OTP verification endpoints must be protected like login endpoints
  • Small misconfigurations can lead to full account compromise

Final Thoughts

This experience reinforced a core security principle:

Attackers don't break systems — they abuse what's left unprotected.

Sometimes, all it takes is opening Burp, reviewing the flow, and asking: "What prevents this from being abused at scale?"

Test responsibly. Stay ethical. 🛡️