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Remote File Inclusion

Need​

Prevention of remote file inclusion attacks

Context​

  • Usage of Node.js v14.0.0 for server-side JavaScript development
  • Usage of Express for building web applications and handling HTTP requests

Description​

Non compliant code​

import express from 'express';

const app = express();

app.get('/loadFile', (req, res) => {
  const fileName = req.query.file;
  const filePath = `/path/to/files/${fileName}`;

  // Vulnerable code: Loading and executing file from remote location
  require(filePath);

  res.send('File loaded successfully');
});

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

The vulnerability in this code is a Remote File Inclusion (RFI) vulnerability. It occurs in the app.get('/loadFile') route handler.

The vulnerability arises from the usage of the require() function with user-supplied input (filePath). The require() function is used to load and execute a file in Node.js. In this case, the code constructs the file path using the user-provided fileName from the query parameter.

An attacker can exploit this vulnerability by manipulating the fileName parameter and supplying a remote file location. For example, an attacker could provide a value like http://attacker.com/malicious.js as the fileName parameter. When the code executes require(filePath), it will attempt to load and execute the remote file, which could contain malicious code.

This vulnerability allows an attacker to retrieve sensitive data from the server or execute arbitrary commands remotely, depending on the content of the malicious file.

To mitigate this vulnerability, it is recommended to avoid using dangerous functions like require() with unsanitized user input. Instead, the code should validate and sanitize the user input before using it in file operations.

Steps​

  • Sanitize the input: Before using the file name obtained from the request query parameter, ensure that it is properly sanitized to prevent any malicious file inclusion. You can use a library or custom function to sanitize the file name.
  • Validate the file name: Check if the file name obtained from the request query parameter is valid and allowed. You can define a whitelist of allowed file names or use regular expressions to validate the file name.
  • Use a secure file loading mechanism: Instead of using the 'require' function to load and execute the file, consider using a more secure method such as 'fs.readFile' or 'fs.readFileSync' to read the file contents and then evaluate or execute it.
  • Restrict file access: Ensure that the file path is restricted to a specific directory and does not allow access to sensitive files or directories outside of the intended scope.
  • Implement proper error handling: Handle any errors that may occur during file loading or execution and provide appropriate error messages without exposing sensitive information.
  • Log and monitor file loading activities: Implement logging and monitoring mechanisms to track and detect any suspicious or unauthorized file loading attempts.
  • Regularly update and patch dependencies: Keep all dependencies, including the Express framework, up to date with the latest security patches to mitigate any potential vulnerabilities.

Compliant code​

import express from 'express';
import fs from 'fs';

const app = express();

app.get('/loadFile', (req, res) => {
  const fileName = sanitizeFileName(req.query.file);
  const filePath = `/path/to/files/${fileName}`;

  if (isValidFileName(fileName)) {
    fs.readFile(filePath, 'utf8', (err, fileContent) => {
      if (err) {
        console.error(err);
        res.status(500).send('Error loading file');
      } else {
        try {
          eval(fileContent);
          res.send('File loaded and executed successfully');
        } catch (error) {
          console.error(error);
          res.status(500).send('Error executing file');
        }
      }
    });
  } else {
    res.status(400).send('Invalid file name');
  }
});

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

function sanitizeFileName(fileName: string): string {
  // Implement your sanitization logic here
  // Example: return fileName.replace(/[^a-zA-Z0-9.-]/g, '');
  return fileName;
}

function isValidFileName(fileName: string): boolean {
  // Implement your validation logic here
  // Example: return /^[a-zA-Z0-9.-]+$/.test(fileName);
  return true;
}

The fixed code addresses the vulnerability by implementing several measures to ensure the safe loading and execution of files.

  1. The code uses the Express framework to handle HTTP requests and responses.

  2. The sanitizeFileName function is introduced to sanitize the file name received as a query parameter. This function can be customized to implement specific sanitization logic, such as removing any characters that are not alphanumeric, dots, or hyphens. In the example code, the function simply returns the original file name without any modifications.

  3. The isValidFileName function is used to validate the file name. It can be customized to implement specific validation rules, such as checking if the file name consists of only alphanumeric characters, dots, or hyphens. In the example code, the function always returns true, indicating that any file name is considered valid.

  4. The code constructs the file path by appending the sanitized file name to a base directory path. This ensures that the file being accessed is within the expected directory and prevents directory traversal attacks.

  5. The code uses the fs.readFile function to read the contents of the file asynchronously. It specifies the 'utf8' encoding to read the file as a text file.

  6. Inside the callback function of fs.readFile, the code checks for any errors that occurred during file reading. If an error occurs, it logs the error and sends a 500 status response indicating an error loading the file.

  7. If the file is successfully read, the code uses the eval function to execute the file content. However, it is important to note that using eval can be risky and should be avoided whenever possible. In a real-world scenario, it is recommended to use alternative methods to execute the file content, such as a sandboxed environment or a specific interpreter for the file type.

  8. If the file execution is successful, the code sends a 200 status response with a success message. If an error occurs during file execution, it logs the error and sends a 500 status response indicating an error executing the file.

  9. If the file name is determined to be invalid based on the isValidFileName function, the code sends a 400 status response indicating an invalid file name.

  10. The code listens for incoming requests on port 3000 and logs a message when the server starts successfully.

Overall, the fixed code implements input sanitization, file path validation, and error handling to mitigate the vulnerability of remote file inclusion. However, it is important to note that the usage of eval should be avoided whenever possible, and alternative methods should be considered for executing file content.

References​

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