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Authentication mechanism absence or evasion - OTP

Need​

Implementation of a robust and secure OTP (One-Time Password) authentication mechanism

Context​

  • Usage of Dart 2.0 for building high-performance web and mobile applications
  • Usage of Shelf for managing and organizing data in Node.js applications

Description​

Non compliant code​

import 'package:shelf/shelf.dart' as shelf;

shelf.Response handleRegister(shelf.Request request) {
  var params = request.context['params'];
  String verificationCode = params['verificationCode'];

  if (verificationCode == null) {
    return shelf.Response.forbidden('Verification code is required');
  }
  
  // Vulnerability: No verification of the OTP code is performed.
  // Any code, regardless of length or content, is accepted.

  // Continue with registration process...
  return shelf.Response.ok('Registration successful');
}

In the above code, the handleRegister function is used to handle new user registrations. The function retrieves the verificationCode from the request parameters, which is supposed to be the OTP (One Time Password) sent to the user's mobile number.

The vulnerability lies in the fact that there is no validation performed on the verificationCode. The function only checks if the verificationCode is null or not. If it is not null, the function proceeds with the registration process, regardless of the content of the verificationCode. This means that any value, even if it doesn't match the expected 6-digit OTP, is accepted.

This allows an attacker to bypass the OTP verification step by providing any non-null value for the verificationCode. This could lead to unauthorized registrations, and potentially allow an attacker to create accounts for other users without their knowledge or consent.

Steps​

  • Create a function to validate the OTP code. This function should check if the code is not null, has the correct length, and matches the code sent to the user's phone.
  • Call this validation function in the registration process, before proceeding with the rest of the registration.
  • If the validation function returns false, return an error response indicating that the verification code is incorrect.
  • Store the OTP code securely on the server side and compare the user input with the stored code.
  • Implement a mechanism to limit the number of attempts a user can make to enter the OTP code to prevent brute force attacks.
  • Ensure that the OTP code expires after a certain period of time for added security.

Compliant code​

import 'package:shelf/shelf.dart' as shelf;

// Function to validate the OTP code
bool validateOTP(String userOTP, String serverOTP) {
  if (userOTP == null || userOTP.length != 6 || userOTP != serverOTP) {
    return false;
  }
  return true;
}

shelf.Response handleRegister(shelf.Request request) {
  var params = request.context['params'];
  String verificationCode = params['verificationCode'];
  String serverOTP = params['serverOTP']; // This should be fetched from the server

  if (!validateOTP(verificationCode, serverOTP)) {
    return shelf.Response.forbidden('Invalid verification code');
  }

  // Continue with registration process...
  return shelf.Response.ok('Registration successful');
}

The updated code includes a function validateOTP that validates the OTP code entered by the user. This function checks if the code is not null, has the correct length of 6 digits, and matches the OTP code stored on the server.

In the handleRegister function, before proceeding with the registration process, the validateOTP function is called with the user's verification code and the server's OTP code as arguments. If the validateOTP function returns false, an error response is returned indicating that the verification code is incorrect.

This code assumes that the OTP code is securely stored on the server and is fetched as serverOTP when needed. It also assumes that mechanisms are in place to limit the number of attempts a user can make to enter the OTP code and to ensure that the OTP code expires after a certain period of time. These measures help to prevent brute force attacks and enhance the security of the OTP verification process.

References​

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