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Email flooding

Need​

Prevention of email flooding and inbox saturation

Context​

  • Usage of Java for building cross-platform applications
  • Usage of javax.mail for sending and receiving emails in Java applications

Description​

Non compliant code​

import javax.mail.*;
import javax.mail.internet.*;

public class EmailFloodingVulnerability {
    public static void main(String[] args) {
        String to = "[email protected]";
        String from = "[email protected]";
        String host = "localhost";

        Properties properties = System.getProperties();
        properties.setProperty("mail.smtp.host", host);

        Session session = Session.getDefaultInstance(properties);

        try {
            for (int i = 0; i < 10000; i++) {
                MimeMessage message = new MimeMessage(session);
                message.setFrom(new InternetAddress(from));
                message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
                message.setSubject("Flooding Email " + i);
                message.setText("This is a flooding email.");
                Transport.send(message);
            }
        } catch (MessagingException mex) {
            mex.printStackTrace();
        }
    }
}

In the above Java code, we are using the Java Mail API to send emails. The for loop is used to send a large amount of emails (in this case, 10000 emails) to the victim's email address.

The to variable is the email address of the victim, and the from variable is the email address of the attacker. The host variable is the SMTP server that sends the email.

The properties object is used to set the SMTP server host. The session object is used to get the default instance of the current session.

Inside the try block, we are creating a new MimeMessage for each iteration of the loop, setting the sender and recipient email addresses, setting the subject and text of the email, and then sending the email using the Transport.send(message) method.

This code is vulnerable to email flooding, as it allows an attacker to send a large amount of emails to a victim's email address, potentially causing the victim's inbox to be saturated.

To mitigate this vulnerability, it's recommended to add a rate limit to the number of emails that can be sent in a certain period of time, or to add CAPTCHA verification to prevent automated scripts from sending emails.

Steps​

  • Implement rate limiting to restrict the number of emails that can be sent within a certain time period.
  • Add CAPTCHA or other user verification mechanisms to prevent automated flooding of emails.
  • Implement email filtering and spam detection mechanisms to identify and block flooding emails.
  • Monitor email server logs for suspicious activity and implement alerting mechanisms.
  • Educate users about email security best practices and encourage them to report suspicious emails.
  • Regularly update and patch the email server software to fix any known vulnerabilities.

Compliant code​

import javax.mail.*;
import javax.mail.internet.*;

public class EmailFloodingVulnerability {
    private static final int MAX_EMAILS_PER_HOUR = 100; // Set your own limit
    private static int emailCount = 0;
    private static long startTime = System.currentTimeMillis();

    public static void main(String[] args) {
        String to = "[email protected]";
        String from = "[email protected]";
        String host = "localhost";

        Properties properties = System.getProperties();
        properties.setProperty("mail.smtp.host", host);

        Session session = Session.getDefaultInstance(properties);

        try {
            for (int i = 0; i < 10000; i++) {
                long currentTime = System.currentTimeMillis();
                if (currentTime - startTime > 3600000) { // Reset every hour
                    startTime = currentTime;
                    emailCount = 0;
                }

                if (emailCount >= MAX_EMAILS_PER_HOUR) {
                    System.out.println("Email limit reached. Try again later.");
                    break;
                }

                MimeMessage message = new MimeMessage(session);
                message.setFrom(new InternetAddress(from));
                message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
                message.setSubject("Flooding Email " + i);
                message.setText("This is a flooding email.");
                Transport.send(message);
                emailCount++;
            }
        } catch (MessagingException mex) {
            mex.printStackTrace();
        }
    }
}

The above code implements rate limiting to prevent email flooding. The MAX_EMAILS_PER_HOUR constant sets the maximum number of emails that can be sent within an hour. The emailCount variable keeps track of the number of emails sent within the current hour. The startTime variable records the start time of the current hour.

In the for loop, the current time is checked. If an hour has passed since startTime, startTime is reset to the current time and emailCount is reset to 0. If emailCount has reached MAX_EMAILS_PER_HOUR, the program prints a message and breaks out of the loop, preventing any more emails from being sent.

This solution only addresses the rate limiting aspect of the vulnerability. For a more comprehensive solution, consider implementing CAPTCHA or other user verification mechanisms, email filtering and spam detection, monitoring and alerting mechanisms, user education, and regular updates and patches.

References​

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