The Vulnerability: Two-Factor Authentication bypass
the Two-Factor Authentication (2FA) mechanism in an application could be bypassed simply by modifying the server response. This allowed an attacker to gain unauthorized access even if they entered an incorrect OTP!
Proof of Concept (PoC)
1️⃣ Login Attempt
Navigate to the login page and enter valid credentials.
The application prompts for 2FA verification using an authenticator app.
2️⃣ Capture the 2FA Request
Enter an incorrect OTP (e.g., 123456).
Intercept the request using Burp Suite or any proxy tool.
3️⃣ Modify the Server Response
Observe that the server responds with an OTP invalid message.
Modify the response:
✅ Change the 403 Forbidden status to 200 OK.
✅ Remove or modify the error message.
4️⃣ Bypass 2FA
The application incorrectly processes the modified response as successful verification.
The attacker gains full access, bypassing 2FA security entirely!
⚠️ Impact of This Vulnerability
🔴 Account Takeover – Unauthorized access to user accounts.
🔴 Data Breach – Exfiltration of sensitive information.
🔴 Privilege Escalation – If an attacker gains admin access, the damage multiplies.
🔴 Bypassing Security Controls – The 2FA mechanism becomes useless.
🛡️ Mitigation Recommendations
✅ Enforce 2FA on the Server-Side – Don’t rely on client-side validation!
✅ Use Signed or Encrypted Responses – Prevent tampering by cryptographic signing.
✅ Verify 2FA Authentication on the Backend – Before granting access, ensure that 2FA was successfully completed.
✅ Implement Proper Session Handling – Activate sessions only after server-side 2FA validation.
💠Lessons Learned
Even though I didn’t get the bounty, this finding reinforced the importance of server-side security in authentication flows. Security should never rely only on the client side!
BugBounty CyberSecurity EthicalHacking Infosec 2FABypass WebSecurity AppSec
Tags
Web Application