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Symmetric denial of service - SMTP


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


  • Usage of PHP 7.0 for server-side scripting and web development
  • Usage of Request for making HTTP requests
  • Usage of Mail for sending emails in Node.js applications
  • Usage of TestEmail for testing email functionality


Non compliant code

public function sendEmail(Request $request)
$email = $request->txtEmail;
Mail::to($email)->send(new TestEmail());
return response()->json(['message' => 'Email sent successfully']);

This code snippet is a function within a Laravel controller that handles sending emails. The function sendEmail takes a Request object as an argument, which contains the email data sent from the client.

The email address is extracted from the request object with $request->txtEmail. This email address is then passed to the Mail::to() function, which sends an email to the given email address.

The problem with this code is that there is no validation of the email address before it is passed to the Mail::to() function. It is possible for an attacker to send multiple requests to this endpoint with large amounts of data in the txtEmail field, causing a denial of service (DoS) attack by exhausting server resources.

Additionally, there is no limit to the number of requests that can be made to this endpoint, and no limit to the number of characters that can be entered into the txtEmail field, further increasing the risk of a DoS attack.

This vulnerability can be exploited to crash the server, making the SMTP service unavailable for legitimate users. This is a serious issue as it can disrupt the normal operation of the application and potentially lead to data loss or other security issues.


  • Validate the email input to ensure it's a properly formatted email address.
  • Implement rate limiting to prevent a user from sending too many requests in a short period of time.
  • Add a character limit to the txtEmail attribute to prevent excessively long strings.
  • Consider using a CAPTCHA or similar system to ensure that the requests are being made by a human and not a bot.
  • Handle exceptions properly to prevent the server from crashing when an error occurs.

Compliant code

use Illuminate\\Support\\Facades\\Validator;
use Illuminate\\Http\\Exceptions\\ThrottleRequestsException;

public function sendEmail(Request $request)
$rules = [
'txtEmail' => 'required|email|max:255',

$validator = Validator::make($request->all(), $rules);

if ($validator->fails()) {
return response()->json(['error' => $validator->errors()], 400);

try {
$email = $request->txtEmail;
Mail::to($email)->send(new TestEmail());
return response()->json(['message' => 'Email sent successfully']);
} catch (ThrottleRequestsException $e) {
return response()->json(['error' => 'Too many requests'], 429);
} catch (\\Exception $e) {
return response()->json(['error' => 'Server error'], 500);

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

Firstly, we use Laravel's built-in validation to ensure that the txtEmail attribute is a properly formatted email address and does not exceed 255 characters. This is done using the required, email, and max:255 validation rules.

Next, we use a try-catch block to handle exceptions that may occur when sending the email. If a ThrottleRequestsException is thrown, this means that the user has sent too many requests in a short period of time. In this case, we return a 429 (Too Many Requests) response. If any other exception is thrown, we return a 500 (Server Error) response.

This approach ensures that the server does not crash when an error occurs, and also provides feedback to the user about what went wrong. It also helps to prevent a single user from overwhelming the server with requests, which could lead to a denial of service.