In the world of cybersecurity, two-factor authentication (2FA) is a cornerstone of protecting user accounts from unauthorized access. It adds an extra layer of security beyond just a username and password. However, like any system, 2FA can have flaws. One such vulnerability is “Improper Authentication” due to reusable One-Time Passwords (OTPs). This issue occurs when an OTP, meant to be single-use and time-limited, can be reused even after it should have expired.

This article will first explain what reusable 2FA OTP vulnerabilities are, why they happen, and most importantly, how you can find them in web applications. We’ll cover step-by-step methods for detection, tools to use, and best practices for ethical testing. Then, we’ll dive into a specific real-world example: HackerOne report #2529780, submitted by researcher xklepxn in June 2024, which highlighted this issue in HackerOne’s own platform. By the end, you’ll have a comprehensive understanding of the vulnerability, its impacts, and how to mitigate it.