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Lack of data validation - Reflected Parameters

Need​

Implementation of robust data validation and sanitization mechanisms

Context​

  • Usage of Java for building robust and scalable applications
  • Usage of javax.servlet for Java web application development
  • Usage of javax.servlet.http for handling HTTP requests and responses in Java web applications

Description​

Non compliant code​

import javax.servlet.*;
import javax.servlet.http.*;

public class VulnerableServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String parameter = request.getParameter("input");

        PrintWriter out = response.getWriter();
        out.println("<html><body>");
        out.println("<h1>" + parameter + "</h1>");
        out.println("</body></html>");
    }
}

The above code represents a servlet in a Java backend application using the javax.servlet package. It is vulnerable due to a lack of data validation for user input.

The doGet method is responsible for handling GET requests made to the server. It retrieves a parameter named "input" from the request, and directly includes it in the HTML response without any validation or sanitization.

This is problematic because the user input is reflected back in the server's HTTP response. If the user input contains special characters or script tags, it can lead to Cross-Site Scripting (XSS) vulnerabilities. For example, if a user enters "<script>alert('XSS')</script>" as input, the server will reflect this back in the response and the script will be executed in the browser of any user viewing the response. This could lead to various harmful consequences, such as stealing user session data or performing actions on behalf of the user.

Steps​

  • Implement input validation to ensure that user input does not contain special characters.
  • Use proper output encoding to prevent XSS attacks.
  • Consider using a web application firewall (WAF) to provide an additional layer of protection against XSS attacks.
  • Regularly update and patch the server and its dependencies to mitigate any potential vulnerabilities.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import org.owasp.encoder.Encode;

public class SecureServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String parameter = request.getParameter("input");

        // Validate the input to ensure it does not contain special characters
        if (!parameter.matches("[a-zA-Z0-9]*")) {
            throw new ServletException("Invalid input");
        }

        PrintWriter out = response.getWriter();
        out.println("<html><body>");

        // Use OWASP Encoder to prevent XSS attacks
        out.println("<h1>" + Encode.forHtml(parameter) + "</h1>");
        out.println("</body></html>");
    }
}

The above code fixes the vulnerability by implementing input validation and output encoding.

  1. Input Validation: The matches("[a-zA-Z0-9]*") function is used to validate the user input. It checks if the input contains only alphanumeric characters (both lower and upper case). If the input contains any special characters, it throws a ServletException with the message "Invalid input".

  2. Output Encoding: The OWASP Encoder library is used to encode the user input before it is included in the server response. The Encode.forHtml(parameter) function encodes the special characters in the input that can be used to launch XSS attacks. This ensures that the input is safe to include in the HTML response.

Remember to add a Web Application Firewall (WAF) for an additional layer of protection against XSS attacks and regularly update and patch the server and its dependencies to mitigate any potential vulnerabilities.

References​

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