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Use of an insecure channel - SMTP

Need​

Secure transmission of confidential information over SMTP

Context​

  • Usage of C# for developing robust and scalable applications
  • Usage of System.Net.Mail for sending emails in .NET development
  • Usage of System.Net for network communication in .NET development

Description​

Non compliant code​

public void ConfigureSmtp()
{
    SmtpClient client = new SmtpClient("smtp.example.com");
    client.UseDefaultCredentials = false;
    client.Credentials = new NetworkCredential("[email protected]", "password");
    client.Send("[email protected]", "[email protected]", "subject", "body");
}

The above C# code shows a configuration of the SMTP (Simple Mail Transfer Protocol) service in an ASP.NET Core application. The SmtpClient class is used to send emails to an SMTP server for delivery.

The vulnerability here is the use of an insecure communication channel as the SMTP service is not configured to use SSL (Secure Sockets Layer). This means that the data being sent via this channel is not encrypted and can be intercepted and viewed by malicious actors.

The SmtpClient object is created with the SMTP server address "smtp.example.com". The UseDefaultCredentials property is set to false and the Credentials property is set to a new NetworkCredential object with a username and password. This implies that the email is sent with these credentials.

However, without the use of SSL, the credentials, along with any other information in the email (such as the sender, recipient, subject, and body), are sent in plain text over the network. This can potentially expose sensitive information to unauthorized individuals who may be eavesdropping on the network traffic.

In summary, the lack of encryption in the communication channel used by the SMTP service in this code makes it vulnerable to data breaches.

Steps​

  • Enable SSL/TLS encryption for the SMTP service
  • Use secure connection settings when configuring the SmtpClient
  • Ensure that the credentials used for authentication are secure
  • Implement proper error handling and logging for SMTP operations

Compliant code​

public void ConfigureSmtp()
{
    SmtpClient client = new SmtpClient("smtp.example.com");
    client.UseDefaultCredentials = false;
    client.Credentials = new NetworkCredential("[email protected]", "password");
    client.EnableSsl = true; // Enable SSL
    try
    {
        client.Send("[email protected]", "[email protected]", "subject", "body");
    }
    catch (Exception ex)
    {
        // Log exception details
        Console.WriteLine("Exception caught in CreateTestMessage2(): {0}", ex.ToString());
    }
}

The updated code now includes the EnableSsl property set to true which ensures that the connection to the SMTP server is encrypted using SSL/TLS. This prevents any confidential information from being viewed without encryption during transmission.

The SmtpClient.Send method is now wrapped in a try-catch block to handle any exceptions that may occur during the sending of the email. If an exception is caught, it is logged to the console. In a real-world application, you would likely want to log this to a file or a logging service instead.

The NetworkCredential object is used to specify the credentials for the SMTP server. It's important to ensure that these credentials are stored securely and not hard-coded into the application as they are here. Consider using a secure method of storing these credentials, such as a secure server or environment variables.

References​

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