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Weak CAPTCHA

Need​

Implementation of a strong and secure CAPTCHA mechanism

Context​

  • Usage of Java 1.0 for developing applications and software solutions
  • Usage of javax.servlet.http.HttpServletRequest for handling HTTP requests in Java Servlets
  • Usage of HttpServletResponse for handling HTTP responses in Java Servlets
  • Usage of javax.servlet.ServletException for handling servlet exceptions in Java web applications
  • Usage of java.io.IOException for handling input/output errors in Java programming

Description​

Non compliant code​

import javax.servlet.http.*;
import java.io.IOException;

public class CaptchaServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String captcha = request.getParameter("captcha");
        String sessionCaptcha = (String) request.getSession().getAttribute("captcha");

        if (captcha != null && captcha.equals(sessionCaptcha)) {
            // CAPTCHA is valid, proceed with the request
        } else {
            // CAPTCHA is invalid, return an error
        }
    }
}

The above code is a simple implementation of a CAPTCHA verification system that is used in a Java Servlet. The Servlet accepts a POST request that contains the CAPTCHA value inputted by the user, and compares it with the CAPTCHA value stored in the user's session.

The vulnerability lies in the fact that the CAPTCHA verification is done on the server-side without any additional security measures. This allows a bot to use optical recognition tools to read the CAPTCHA value and pass it in the POST request, thereby bypassing the CAPTCHA verification.

The doPost method retrieves the CAPTCHA value from the request parameters and the session attributes. If the two values match, it assumes that the CAPTCHA is valid. However, it does not check if the request was made by a human or a bot. This makes the system susceptible to automated attacks.

Steps​

  • Replace the current CAPTCHA implementation with a stronger and more secure one.
  • Use a CAPTCHA implementation that is resistant to optical recognition tools.
  • Implement additional security measures to prevent automated attacks, such as rate limiting or IP blocking.
  • Regularly update and maintain the CAPTCHA implementation to stay ahead of emerging vulnerabilities and attack techniques.
  • Consider implementing multi-factor authentication or other additional security measures to further protect against automated attacks.

Compliant code​

import com.google.code.kaptcha.Constants;
import com.google.code.kaptcha.Producer;
import com.google.code.kaptcha.servlet.KaptchaServlet;
import javax.servlet.http.*;
import java.io.IOException;

public class CaptchaServlet extends KaptchaServlet {
    private static final long serialVersionUID = 1L;
    private Producer captchaProducer = null;

    public void init() throws ServletException {
        super.init();
        captchaProducer = (Producer) getServletContext().getAttribute(Constants.KAPTCHA_PRODUCER_IMPL);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String captcha = request.getParameter("captcha");
        String sessionCaptcha = (String) request.getSession().getAttribute(Constants.KAPTCHA_SESSION_KEY);

        if (captcha != null && captcha.equals(sessionCaptcha)) {
            // CAPTCHA is valid, proceed with the request
        } else {
            // CAPTCHA is invalid, return an error
        }
    }
}

The above code fixes the vulnerability by replacing the insecure CAPTCHA implementation with a more secure one using the Kaptcha library. Kaptcha is a popular Java library for generating CAPTCHAs that are resistant to optical recognition tools.

In the init() method, we initialize the captchaProducer object by getting the Kaptcha producer implementation from the servlet context.

In the doPost() method, we get the CAPTCHA value from the session using the Kaptcha session key. This value is then compared with the CAPTCHA value entered by the user.

If the two values match, the CAPTCHA is valid and the request is processed. If they don't match, the CAPTCHA is invalid and an error is returned.

This implementation is more secure against automated attacks and optical recognition tools. However, it's important to implement additional security measures such as rate limiting or IP blocking, and to regularly update and maintain the CAPTCHA implementation to stay ahead of emerging vulnerabilities and attack techniques.

Additionally, consider implementing multi-factor authentication or other additional security measures to further protect against automated attacks.

References​

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