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Lack of data validation - Source Code

Need​

Implementation of robust data validation in the source code

Context​

  • Usage of Java 8 for developing applications with enhanced features and performance
  • Usage of javax.servlet-api for developing Java web applications with Servlets

Description​

Non compliant code​

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.regex.Pattern;

public class UserServlet extends HttpServlet {
    private static final Pattern USERNAME_PATTERN = Pattern.compile("^[a-zA-Z0-9]*$");

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        
        if (!USERNAME_PATTERN.matcher(username).matches()) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid username");
            return;
        }
        
        // Process the username
    }
}

In the above code, the doPost method retrieves a username parameter from the HTTP request and checks if it matches a regular expression pattern. This pattern only allows alphanumeric characters (both lowercase and uppercase).

The vulnerability exists in the use of the regular expression ^[a-zA-Z0-9]*$. Although it seems harmless as it only matches alphanumeric characters, it can lead to a Regular Expression Denial of Service (ReDoS) attack.

In a ReDoS attack, an attacker sends a specially crafted string that takes a long time to evaluate against the regular expression. This can cause the server to become unresponsive or crash, as it spends all its resources evaluating the regular expression.

For example, an attacker could send a long string of valid characters followed by an invalid character. The regular expression engine would take a long time to determine that this string does not match the pattern, as it has to backtrack after every unsuccessful match attempt.

The lack of input validation and sanitization further exacerbates the vulnerability. The code does not limit the length of the username parameter, so an attacker could send an extremely long string to overwhelm the server.

To mitigate this vulnerability, you should sanitize and validate all user inputs. Consider limiting the length of the username parameter, and use a more efficient regular expression or a different method to validate the username.

Steps​

  • Implement proper input validation to ensure that only valid data is accepted.
  • Avoid using complex regular expressions that can be exploited by attackers.
  • Consider using a whitelist approach to validate input, allowing only specific characters or patterns.
  • Use a well-tested and secure library or framework for input validation.
  • Regularly update and patch the software to ensure any known vulnerabilities are addressed.

Compliant code​

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import org.apache.commons.validator.routines.RegexValidator;

public class UserServlet extends HttpServlet {
    private static final String USERNAME_PATTERN = "^[a-zA-Z0-9]*$";
    private static final RegexValidator validator = new RegexValidator(USERNAME_PATTERN);

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        
        if (!validator.isValid(username)) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid username");
            return;
        }
        
        // Process the username
    }
}

The updated code now uses the Apache Commons Validator library, which is a well-tested and secure library for input validation. The RegexValidator class is used to validate the username against the specified pattern.

The pattern used for validation is a simple one, allowing only alphanumeric characters. This is a whitelist approach to input validation, which is generally safer than a blacklist approach.

The isValid() method of the RegexValidator class is used to check if the username matches the pattern. If the username is invalid, an error is sent in the response and the method returns immediately, preventing any further processing of the invalid username.

This approach ensures that only valid data is accepted, reducing the risk of attacks that exploit complex regular expressions. It also simplifies the code and makes it easier to maintain and update. Regular updates and patches should be applied to the Apache Commons Validator library to address any known vulnerabilities.

References​

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