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Unauthorized access to files

Need​

Prevention of unauthorized access to files and functionalities

Context​

  • Usage of Java for building cross-platform applications
  • Usage of javax.servlet for building Java web applications with Servlet technology
  • Usage of java.io for input and output operations in Java

Description​

Non compliant code​

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

public class UserDataServlet extends HttpServlet {

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userId = request.getParameter("userId");
        String userData = request.getParameter("userData");

        File userFile = new File("/sharepoint/" + userId + ".txt");
        if(!userFile.exists()) {
            userFile.createNewFile();
        }

        FileWriter writer = new FileWriter(userFile);
        writer.write(userData);
        writer.close();
    }
}

The above code is a simple servlet that accepts POST requests to update user data. The user ID and data are passed as parameters in the request. The servlet then writes this data to a file in the Sharepoint directory.

The vulnerability lies in the fact that the user ID is directly used to create a new file without any validation or sanitization. A malicious user can pass special characters or path sequences (like "../") in the user ID to traverse the directory structure and create or overwrite files outside the intended directory, leading to unauthorized file access.

Moreover, the user data is also directly written to the file without any validation or sanitization, opening up possibilities for various injection attacks. For example, if the files are later served or processed in a way that interprets certain sequences as commands or special symbols, this could lead to further security issues.

Steps​

  • Sanitize user input: Implement input validation and filtering to ensure that only safe and expected characters are allowed in the userId and userData parameters.
  • Use proper file path handling: Avoid directly concatenating user input with file paths. Instead, use a secure method to construct the file path, such as using a whitelist of allowed characters or encoding the user input.
  • Implement access control: Ensure that the user has the necessary authorization to access and modify the user data files. Use authentication and authorization mechanisms to enforce access control.
  • Use secure file permissions: Set appropriate file permissions to restrict access to the user data files. Only allow authorized users or processes to read or modify the files.
  • Regularly update and patch dependencies: Keep all libraries and frameworks used in the application up to date to mitigate any known vulnerabilities that could be exploited.
  • Implement logging and monitoring: Monitor and log any unauthorized access attempts or suspicious activities related to the user data files. This can help in detecting and responding to potential security breaches.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.nio.file.*;
import java.util.regex.*;

public class UserDataServlet extends HttpServlet {

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

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userId = request.getParameter("userId");
        String userData = request.getParameter("userData");

        // Validate user input
        if (!VALID_USER_ID.matcher(userId).matches()) {
            throw new ServletException("Invalid user ID");
        }

        // Construct file path securely
        Path userFilePath = Paths.get("/sharepoint", userId + ".txt").normalize();

        // Check if user has necessary authorization
        if (!isUserAuthorized(userId)) {
            throw new ServletException("Unauthorized access");
        }

        // Create file if it doesn't exist
        if (!Files.exists(userFilePath)) {
            Files.createFile(userFilePath);
        }

        // Write user data to file
        Files.write(userFilePath, userData.getBytes(), StandardOpenOption.WRITE);

        // Set secure file permissions
        setSecurePermissions(userFilePath);
    }

    private boolean isUserAuthorized(String userId) {
        // Implement authentication and authorization checks
        return true;
    }

    private void setSecurePermissions(Path userFilePath) throws IOException {
        // Set file permissions to restrict access
        Files.setPosixFilePermissions(userFilePath, PosixFilePermissions.fromString("rw-------"));
    }
}

The updated code includes several security improvements:

  1. Input Validation: The user ID is now validated against a regular expression that only allows alphanumeric characters. This prevents the injection of potentially dangerous characters.

  2. Secure File Path Handling: The file path is now constructed using the Paths.get() method, which safely combines the base directory with the user ID. The normalize() method is used to resolve any ".." sequences to prevent directory traversal attacks.

  3. Access Control: The isUserAuthorized() method is used to check if the user has the necessary authorization to access and modify the user data files. This method should be implemented to perform the actual authentication and authorization checks.

  4. Secure File Permissions: The setSecurePermissions() method is used to set the file permissions to "rw-------", which means that only the owner of the file can read and write to it.

  5. Secure File Writing: The Files.write() method is used to write the user data to the file. This method opens the file for writing, writes the data, and then closes the file. This is safer than using a FileWriter because it ensures that the file is properly closed even if an exception occurs.

  6. Regular Updates and Patching: Although not shown in the code, it is recommended to regularly update and patch all libraries and frameworks used in the application to mitigate any known vulnerabilities.

  7. Logging and Monitoring: Although not shown in the code, it is recommended to implement logging and monitoring of any unauthorized access attempts or suspicious activities related to the user data files.

References​

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