Skip to main content

Lack of data validation - Special Characters

Need​

Implementation of robust data validation to prevent the entry of special characters in fields where they are not allowed.

Context​

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

Description​

Non compliant code​

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

public class DataServlet extends HttpServlet {

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String firstName = request.getParameter("firstName");
        String lastName = request.getParameter("lastName");

        // Process the data...
    }
}

This Java servlet receives POST requests and extracts the firstName and lastName parameters from the request. These parameters are then used in the further processing of the data.

However, there is a significant vulnerability in this code: it does not validate the input data. As a result, a user can enter any data into the firstName and lastName fields, including special characters.

This lack of validation can lead to several problems:

  1. Injection attacks: If these parameters are used to construct a database query or command, special characters can be used to alter the meaning of the command, leading to an injection attack.

  2. Cross-site Scripting (XSS) attacks: If these parameters are displayed in a web page without proper escaping, special characters can be used to inject malicious scripts into the page.

  3. Data corruption: If these parameters are used to construct a filename or other data structure, special characters can cause unexpected behavior or even data corruption.

Therefore, it is crucial to validate all input data to ensure it does not contain any special characters or any other unexpected data.

Steps​

  • Implement input validation to check for special characters in the firstName and lastName fields.
  • Use regular expressions or a library to define a pattern for valid input.
  • Reject or sanitize any input that contains special characters.
  • Provide appropriate error messages or feedback to the user if invalid input is detected.
  • Consider implementing server-side validation in addition to client-side validation to ensure data integrity.
  • Regularly update and maintain the input validation mechanism to account for new types of special characters or vulnerabilities.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import java.util.regex.Pattern;

public class DataServlet extends HttpServlet {

    private static final Pattern VALID_INPUT = Pattern.compile("[a-zA-Z0-9]*");

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String firstName = request.getParameter("firstName");
        String lastName = request.getParameter("lastName");

        if (!isValid(firstName) || !isValid(lastName)) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid input: only alphanumeric characters are allowed.");
            return;
        }

        // Process the data...
    }

    private boolean isValid(String input) {
        return VALID_INPUT.matcher(input).matches();
    }
}

The updated code includes a method isValid(String input) that uses a regular expression to check if the input string contains only alphanumeric characters (letters and numbers). The regular expression [a-zA-Z0-9]* matches any string that consists of zero or more (*) alphanumeric characters.

In the doPost method, before processing the data, we now check if the firstName and lastName parameters are valid by calling isValid(firstName) and isValid(lastName). If either of these checks fails, we send an error response with the status code SC_BAD_REQUEST (400) and a descriptive error message, and then return to exit the method.

This way, we ensure that no special characters are included in the firstName and lastName fields, thus preventing potential security issues related to unvalidated input.

Remember that this is server-side validation. It is also recommended to implement client-side validation to provide immediate feedback to the user and reduce unnecessary server requests. However, client-side validation can be bypassed, so server-side validation is still necessary to ensure data integrity.

References​

Last updated on