Preventing Path Manipulation in .appconfig File Retrieval

TL;DR

Path manipulation vulnerabilities happen when an attacker can control how your application finds and opens files, potentially letting them access sensitive data or run malicious code. When reading file paths from a configuration file like .appconfig, always validate the path to ensure it points to where you expect. This guide shows you how.

How Path Manipulation Works

Imagine your application reads a filename from an .appconfig file and then opens that file. If an attacker can change the value in the .appconfig file, they could point it to a different file on the system – maybe one containing harmful code or sensitive information.

Solution: Validate File Paths

1. Understand the Base Path: Determine the expected directory where your configuration files (and therefore the files you’ll be opening) should reside. This is crucial for validation.
   • Example: If your .appconfig file is in the application’s root directory, that’s your base path.
2. Read the Path from .appconfig: Load the filename or filepath from your configuration file.

   string filePath = ConfigurationManager.AppSettings["MyFilePath"];

3. Absolute Path Conversion: Convert any relative paths to absolute paths immediately after reading them.

   string absolutePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, filePath);

4. Validate the Absolute Path: This is the most important step! Check that the resulting path starts with your expected base directory.

   if (!absolutePath.StartsWith(AppDomain.CurrentDomain.BaseDirectory)){
     // Log the attempt and handle it as a security issue
     throw new SecurityException("Invalid file path provided.");
   }

5. Check for Forbidden Characters: Even after checking the base directory, make sure the path doesn’t contain characters that could be used to escape directories (e.g., ..).

   string invalidChars = "../";
   if (absolutePath.Contains(invalidChars)){
     // Log and handle as a security issue
     throw new SecurityException("Invalid file path characters detected.");
   }

6. File Existence Check (Optional): While not always necessary, you can verify that the file actually exists before attempting to open it. This prevents errors but doesn’t guarantee security on its own.

   if (!File.Exists(absolutePath)){
     // Log and handle - maybe a configuration error
     throw new FileNotFoundException("File not found.");
   }

7. Use Safe File Access Methods: When opening the file, use methods that don’t allow arbitrary code execution. Avoid using functions that could interpret the file content as code if you only intend to read data.
   • For example, prefer reading text files line by line instead of executing them.

Example Scenario

Let’s say your application needs to read a configuration file named data.txt from the config subdirectory within the application’s root directory.

1. Base Path: AppDomain.CurrentDomain.BaseDirectory
2. Configuration File (.appconfig): MyDataFile=../sensitive/data.txt (attacker-controlled)
3. Absolute Path Conversion: absolutePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "../sensitive/data.txt")
4. Validation: The validation check will fail because absolutePath does not start with AppDomain.CurrentDomain.BaseDirectory.

Important Considerations

• Logging: Always log any attempts to access files outside of the expected directory. This helps you detect and respond to attacks.
• Principle of Least Privilege: Ensure your application runs with the minimum necessary permissions. If it doesn’t need access to certain directories, don’t grant it that access.
• Regular Security Audits: Regularly review your code and configuration files for potential vulnerabilities.