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Inappropriate coding practices - invalid file

Need​

Enforcement of proper file validation and verification

Context​

  • Usage of Dart 2.12.0 for developing cross-platform applications
  • Usage of Shelf for managing database queries and data modeling
  • Usage of shelf_io for managing and organizing data in a Node.js application

Description​

Non compliant code​

import 'dart:convert';
import 'dart:io';
import 'package:shelf/shelf.dart';
import 'package:shelf/shelf_io.dart' as io;

void main() async {
  var server = await io.serve(handler, 'localhost', 8080);

  print('Serving at http://${server.address.host}:${server.port}');
}

Response handler(Request request) {
  var file = File('file.txt'); // Invalid file
  var content = file.readAsStringSync();

  return Response.ok(content);
}

The above Dart code is a basic backend application that uses the Shelf framework to serve HTTP requests. It has a vulnerability related to inappropriate coding practices specifically related to handling files.

In the handler function, the code reads a file named 'file.txt' synchronously and sends its content as a response to any HTTP request. The vulnerability here is that the file content is read directly and sent without any verification of its content or its extension.

This is a significant security vulnerability because it allows for potential security risks such as serving malicious content or sensitive information unintentionally. For example, if 'file.txt' was replaced with a file containing malicious scripts or sensitive data, this code would serve that content to any client making a request.

Moreover, the code assumes that the file 'file.txt' will always be present in the directory where the server is running. If the file does not exist, the server will throw an unhandled exception, and the application will crash.

To fix this vulnerability, the code should verify the file's content and its extension before reading and serving it. It should also handle the case where the file does not exist gracefully.

Steps​

  • Check if the file exists before trying to read it. This can be done using the 'exists' method of the 'File' class in Dart.
  • Validate the file content against the expected format or structure. If the file is supposed to contain JSON, for example, try parsing it and handle any errors that occur.
  • Consider using a safer method to read the file, such as 'readAsString' which returns a Future. This allows you to handle any errors that occur during the file reading process in a non-blocking way.
  • If the file is not supposed to be part of the repository, remove it and add it to the .gitignore file to prevent it from being accidentally committed again.
  • If the file is supposed to be part of the repository but its content is not compatible with its extension, consider changing the file extension to match the content, or changing the content to match the extension.

Compliant code​

import 'dart:convert';
import 'dart:io';
import 'package:shelf/shelf.dart';
import 'package:shelf/shelf_io.dart' as io;

void main() async {
  var server = await io.serve(handler, 'localhost', 8080);

  print('Serving at http://${server.address.host}:${server.port}');
}

Future<Response> handler(Request request) async {
  var file = File('file.txt'); // Invalid file

  if (await file.exists()) {
    try {
      var content = await file.readAsString();
      var jsonContent = jsonDecode(content); // Assuming the file should contain JSON
      return Response.ok(content);
    } catch (e) {
      return Response.internalServerError(body: 'Invalid file content');
    }
  } else {
    return Response.notFound('File not found');
  }
}

The updated code now checks if the file exists before trying to read it. This is done using the exists method of the File class in Dart. If the file does not exist, a 404 Not Found response is returned.

If the file does exist, the code attempts to read the file content as a string using the readAsString method, which returns a Future. This allows the code to handle any errors that occur during the file reading process in a non-blocking way.

The code then attempts to parse the file content as JSON, assuming that the file should contain JSON. If the parsing is successful, the file content is returned in the response. If an error occurs during parsing, a 500 Internal Server Error response is returned with a message indicating that the file content is invalid.

This approach ensures that the file content is compatible with its extension and that any errors that occur during the file reading or parsing process are handled appropriately. It also prevents the server from crashing due to unhandled exceptions.

References​

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