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Lack of data validation - Emails

Need​

Implementation of email validation to prevent registration with disposable mailboxes

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of System for low-level platform operations and utilities
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core MVC
  • Usage of MyWebApplication.Models for defining and managing models in a web application

Description​

Non compliant code​

using System;
using Microsoft.AspNetCore.Mvc;
using MyWebApplication.Models;

public class AccountController : Controller
{
    private readonly IUserService _userService;
    
    public AccountController(IUserService userService)
    {
        _userService = userService;
    }

    [HttpPost]
    public IActionResult Register(UserRegistrationModel model)
    {
        if (ModelState.IsValid)
        {
            var result = _userService.RegisterUser(model.Email, model.Password);
            
            if (result)
            {
                return RedirectToAction("Index", "Home");
            }
            else
            {
                ModelState.AddModelError("", "Registration failed");
                return View(model);
            }
        }
        
        return View(model);
    }
}

This code represents a simple registration controller in an ASP.NET Core application. The Register action method is responsible for registering a new user.

The vulnerability lies in the fact that there's no validation of the email address provided by the user. More specifically, this code does not check if the email belongs to a disposable mailbox like yopmail.

The Register method simply checks if the model state is valid (i.e., all required fields are filled and all data types are correct) and then proceeds to register the user by calling the RegisterUser method of the IUserService service. The RegisterUser method is assumed to simply add the user to the database and return a boolean indicating the success of the operation.

This lack of validation allows a malicious user to register with a disposable email address, which can lead to several issues such as spamming, lack of user accountability, and inability to contact the user.

Steps​

  • Implement server-side email validation to check if the email belongs to a disposable mailbox provider.
  • Use a reliable email validation library or service to validate the email address.
  • Consider implementing a custom email validation logic to check for disposable email domains.
  • Display appropriate error messages to the user if the email is found to be from a disposable mailbox provider.
  • Consider implementing additional security measures such as rate limiting or CAPTCHA to prevent abuse of the registration process.

Compliant code​

using System;
using Microsoft.AspNetCore.Mvc;
using MyWebApplication.Models;
using MyWebApplication.Services;

public class AccountController : Controller
{
    private readonly IUserService _userService;
    private readonly IEmailValidationService _emailValidationService;
    
    public AccountController(IUserService userService, IEmailValidationService emailValidationService)
    {
        _userService = userService;
        _emailValidationService = emailValidationService;
    }

    [HttpPost]
    public IActionResult Register(UserRegistrationModel model)
    {
        if (ModelState.IsValid)
        {
            // Validate the email address
            var isEmailValid = _emailValidationService.Validate(model.Email);
            if (!isEmailValid)
            {
                ModelState.AddModelError("Email", "Invalid email address. Please use a non-disposable email address.");
                return View(model);
            }

            var result = _userService.RegisterUser(model.Email, model.Password);
            
            if (result)
            {
                return RedirectToAction("Index", "Home");
            }
            else
            {
                ModelState.AddModelError("", "Registration failed");
                return View(model);
            }
        }
        
        return View(model);
    }
}

The updated code now includes an additional service IEmailValidationService which is used to validate the email address provided by the user during registration. This service should contain the logic to check if the email address belongs to a disposable mailbox provider.

In the Register method, before registering the user, we now call the Validate method of the IEmailValidationService to validate the email address. If the email address is found to be invalid (i.e., it belongs to a disposable mailbox provider), we add an error to the ModelState and return the registration view with the model, displaying the error message to the user.

This way, we ensure that only valid, non-disposable email addresses are used for user registration, thereby mitigating the risk of abuse of the registration process.

Please note that the implementation of IEmailValidationService is not shown here. You can use a reliable email validation library or service, or implement your own custom logic to check for disposable email domains. Additionally, consider implementing additional security measures such as rate limiting or CAPTCHA to further prevent abuse of the registration process.

References​

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