In the world of cybersecurity, even the smallest implementation flaw can lead to serious consequences. During a recent authorized penetration test, I discovered a vulnerability in a production web application that allowed unauthorized access to user data — all due to a minor but dangerous oversight in how OTP (One-Time Password) verification was implemented.
🧪 What Was the Vulnerability?
The application had a user lookup feature that allowed visitors to enter a unique ID to retrieve their personal information. Here’s how the intended flow was designed:
- The user enters their ID on a form.
- The application automatically fetched and displayed the registered mobile number associated with that ID (on the frontend).
- An OTP was sent to that number for verification before granting access to sensitive profile data.
On the surface, it all looked secure. But behind the scenes, a critical trust assumption was being made.
🚨 The Exploit: Frontend Trust Gone Wrong
While analyzing the request flow, I intercepted the network traffic using tools like Burp Suite. Surprisingly, I noticed that:
- The mobile number was being sent from the frontend back to the backend along with the OTP request.
- The backend didn’t validate whether the submitted number actually belonged to the ID provided.
This meant I could:
✅ Enter a valid user ID
✅ Modify the mobile number in the request payload
✅ Receive the OTP on my own device
✅ Bypass verification and access someone else’s data
❗ Why Is This Dangerous?
This kind of vulnerability completely undermines the purpose of OTP verification. OTPs are meant to serve as a second layer of authentication — but if an attacker can decide where they go, the system is no longer secure.
Potential Risks:
- Account Takeover: Malicious users can hijack legitimate accounts.
- Identity Theft: Access to personal data can be used for phishing or impersonation.
- Regulatory Violations: Depending on the data accessed, this could breach privacy laws like GDPR or CCPA.
- Brand Damage: A single report of user data being compromised can erode customer trust.
✅ Recommendations for Fix
To remediate this vulnerability, here are best practices that should be followed:
- Never Trust the Frontend: Critical data like phone numbers should be pulled server-side.
- Server-Side Validation: Ensure that the OTP is sent only to the mobile number actually linked to the user ID in your records.
- Audit Logs: Maintain logs of OTP request activities to identify anomalies.
- Rate Limiting: Prevent brute-force attacks by rate limiting OTP requests per user/IP.
- Use Unique OTP Channels: Consider time-based tokens or app-based verification methods for higher security.