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Lack of data validation - Non Sanitized Variables

Need​

Implementation of proper data validation and sanitization techniques

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
  • Usage of java.io for input and output operations in Java

Description​

Non compliant code​

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

public class VulnerableServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userSuppliedData = request.getParameter("data");
        Runtime runtime = Runtime.getRuntime();
        runtime.exec("systemCommand " + userSuppliedData);
    }
}

In the above code, the doPost method of VulnerableServlet class is accepting a request parameter named "data". The value of this parameter is directly used in a system command execution without any sanitization or validation.

This is done by the line runtime.exec("systemCommand " + userSuppliedData);. Here, userSuppliedData is a string that is appended to the systemCommand string.

The problem with this code is that it does not neutralize special elements that could modify the system command. This means that an attacker can provide input in such a way that it alters the system command to perform malicious operations.

For example, if an attacker provides the input as "; rm -rf /", the system command becomes "systemCommand ; rm -rf /". This command first executes the systemCommand and then deletes all files in the root directory.

This vulnerability is known as command injection, which is a type of injection attack where the attacker can execute arbitrary commands on the host operating system.

To mitigate this, it is recommended to avoid the use of external system commands when possible. If it is necessary, then the inputs should be properly sanitized and validated before using them in system commands.

Steps​

  • Implement input validation to ensure that user-supplied data is safe and does not contain any malicious elements.
  • Use parameterized queries or prepared statements when constructing system commands to prevent any form of command injection.
  • Sanitize user-supplied data by removing or escaping any special characters that could modify the system command.
  • Implement proper error handling to handle any unexpected inputs or errors that may occur during the execution of the system command.

Compliant code​

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

public class SecureServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userSuppliedData = request.getParameter("data");
        
        // Sanitize the user supplied data
        String sanitizedData = sanitizeInput(userSuppliedData);
        
        // Use a parameterized command
        String[] command = {"systemCommand", sanitizedData};
        
        Runtime runtime = Runtime.getRuntime();
        try {
            runtime.exec(command);
        } catch (IOException e) {
            // Handle the error
            e.printStackTrace();
        }
    }
    
    // Method to sanitize user input
    private String sanitizeInput(String input) {
        // Remove or escape any special characters
        return input.replaceAll("[^a-zA-Z0-9]", "");
    }
}

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

Firstly, the user-supplied data is sanitized by removing or escaping any special characters that could modify the system command. This is done using the replaceAll method which replaces any non-alphanumeric characters with an empty string.

Secondly, a parameterized command is used when executing the system command. This prevents any form of command injection as the user-supplied data is treated as a single argument to the command and not part of the command itself.

Finally, proper error handling is implemented to handle any unexpected inputs or errors that may occur during the execution of the system command. If an IOException is thrown during the execution of the command, it is caught and the stack trace is printed. This allows for easier debugging and ensures that the program does not crash unexpectedly.

References​

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