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Excessive privileges - Temporary Files

Need​

Restriction of privileges and access modes for temporary files

Context​

  • Usage of Dart for building cross-platform mobile, web, and desktop applications
  • Usage of dart:io for handling input and output operations in Dart programming

Description​

Non compliant code​

import 'dart:io';

void main() {
  var tempDir = Directory.systemTemp.createTempSync();
  var tempFile = File('${tempDir.path}/tempFile.txt');
  
  tempFile.writeAsStringSync('This is a temporary file');
  print('Temporary file created at: ${tempFile.path}');
}

In this Dart code snippet, we are creating a temporary file in the system's temporary directory without properly managing its permissions.

The main function first creates a temporary directory using the Directory.systemTemp.createTempSync() method. This directory is created with default permissions, which are typically very permissive.

Then, a temporary file named tempFile.txt is created in this directory using the File class and the writeAsStringSync method, which writes a string to the file. This file is also created with default permissions.

The problem here is that the application does not restrict the temporary file's privileges or access modes. This means that any user or process on the system can read, write, or delete this file.

This can lead to various security issues. For example, an attacker could replace the contents of the file with malicious data, or they could read sensitive information from the file. Therefore, it's crucial to properly manage the permissions of temporary files in your application.

Steps​

  • Create a unique filename for each temporary file to prevent any potential name collisions.
  • Limit the permissions of the temporary file to only what is necessary. For example, if the file only needs to be read by the application, then it should not have write permissions.
  • Delete the temporary file as soon as it is no longer needed. This reduces the window of opportunity for an attacker to exploit the file.
  • Consider using a secure method to create temporary files, such as using a library or function that generates secure temporary files with restricted permissions.

Compliant code​

import 'dart:io';
import 'dart:math';

void main() {
  var tempDir = Directory.systemTemp.createTempSync();
  var rng = Random();
  var tempFileName = '${tempDir.path}/tempFile_${rng.nextInt(10000)}.txt';
  var tempFile = File(tempFileName);
  
  tempFile.writeAsStringSync('This is a temporary file');
  print('Temporary file created at: ${tempFile.path}');

  // Set the permissions to read-only after writing to the file
  tempFile.setPosixPermissionsSync(FileSystemEntity.permissionRead);

  // Delete the temporary file after use
  tempFile.deleteSync();
}

The updated code now generates a unique filename for each temporary file by appending a random number to the filename. This prevents potential name collisions which could be exploited by an attacker.

After writing to the file, the permissions are set to read-only. This restricts the permissions of the temporary file to only what is necessary, reducing the potential for exploitation.

Finally, the temporary file is deleted as soon as it is no longer needed. This reduces the window of opportunity for an attacker to exploit the file.

By following these steps, the application is less vulnerable to attacks that exploit excessive privileges of temporary files.

References​

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