Skip to main content

Symmetric denial of service - SMTP

Need​

Enhancement of SMTP service to implement proper input validation and limit the number of emails and characters allowed in the txtEmail attribute.

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of System.Net.Mail for sending emails in .NET applications

Description​

Non compliant code​

using System.Net.Mail;

public class EmailController : Controller
{
    [HttpPost]
    public IActionResult SendEmail(string txtEmail)
    {
        try
        {
            var message = new MailMessage();
            message.To.Add(new MailAddress(txtEmail));
            message.From = new MailAddress("[email protected]");
            message.Subject = "Test Email";
            message.Body = "This is a test email";

            using (var smtpClient = new SmtpClient())
            {
                smtpClient.Send(message);
            }

            return Ok("Email sent successfully");
        }
        catch (Exception e)
        {
            return BadRequest(e.Message);
        }
    }
}

The above code represents a simple email sending function in an ASP.NET Core application. The user inputs an email address (txtEmail) to which a test email is sent.

The vulnerability lies in the fact that there is no validation of the txtEmail input. This means that an attacker could potentially send multiple requests with large email addresses or a large number of requests to the SMTP service, leading to a denial of service (DoS) attack.

The server may crash due to the large number of requests or the large size of the requests, as it is not capable of handling such a load. This can lead to unavailability of the service for other users.

This vulnerability can be exploited by an attacker to disrupt the normal functioning of the application, making it unavailable for its intended users.

Steps​

  • Implement input validation for the 'txtEmail' parameter to ensure it meets the required format and length.
  • Set appropriate limits on the number of emails or characters that can be entered.
  • Consider implementing rate limiting or throttling mechanisms to prevent excessive requests to the SMTP service.
  • Handle exceptions gracefully and provide meaningful error messages to the user.
  • Consider implementing logging to track any potential issues or suspicious activities related to the SMTP service.

Compliant code​

using System;
using System.Net.Mail;
using System.Text.RegularExpressions;
using Microsoft.AspNetCore.Mvc;

public class EmailController : Controller
{
    private static readonly Regex EmailRegex = new Regex(@"^[\\w-]+(\\.[\\w-]+)*@([\\w-]+\\.)+[a-zA-Z]{2,7}$");
    private static DateTime LastEmailSentTime;
    private static readonly TimeSpan EmailSendInterval = TimeSpan.FromSeconds(10);

    [HttpPost]
    public IActionResult SendEmail(string txtEmail)
    {
        try
        {
            if (string.IsNullOrEmpty(txtEmail) || txtEmail.Length > 255 || !EmailRegex.IsMatch(txtEmail))
            {
                return BadRequest("Invalid email address");
            }

            if (DateTime.UtcNow - LastEmailSentTime < EmailSendInterval)
            {
                return BadRequest("Too many requests. Please try again later.");
            }

            var message = new MailMessage();
            message.To.Add(new MailAddress(txtEmail));
            message.From = new MailAddress("[email protected]");
            message.Subject = "Test Email";
            message.Body = "This is a test email";

            using (var smtpClient = new SmtpClient())
            {
                smtpClient.Send(message);
                LastEmailSentTime = DateTime.UtcNow;
            }

            return Ok("Email sent successfully");
        }
        catch (Exception e)
        {
            // Log the exception
            Console.WriteLine(e);
            return BadRequest("An error occurred while sending the email. Please try again later.");
        }
    }
}

The updated code includes several measures to prevent a denial of service attack on the SMTP service.

  1. Input Validation: The 'txtEmail' parameter is validated to ensure it is not null or empty, does not exceed 255 characters, and matches the regular expression for a valid email address. If the input is invalid, a "BadRequest" response is returned with an appropriate error message.

  2. Rate Limiting: A static DateTime variable 'LastEmailSentTime' is used to track the time when the last email was sent. Before sending an email, the code checks if the time elapsed since 'LastEmailSentTime' is less than a specified interval (10 seconds in this case). If so, a "BadRequest" response is returned with an error message indicating too many requests.

  3. Exception Handling: The code is wrapped in a try-catch block to handle any exceptions that may occur when sending the email. If an exception is caught, it is logged to the console and a "BadRequest" response is returned with a generic error message.

  4. Logging: Any exceptions that occur are logged to the console. This can help in tracking any potential issues or suspicious activities related to the SMTP service.

References​

Last updated on