Lack of data validation - Path Traversal

Need

Implementation of robust data validation and sanitization mechanisms

Context

• Usage of C# 7.0 for modern and efficient programming in the .NET ecosystem

• Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core

• Usage of Microsoft.AspNetCore.Hosting for hosting ASP.NET Core applications

• Usage of System.IO for input and output operations in .NET development

• Usage of System.IO.MemoryStream for reading and writing data to memory

• Usage of System.IO.FileStream for reading and writing files in .NET

• Usage of System.IO.FileMode for file handling and access control

• Usage of System.IO.Path for manipulating file and directory paths

Description

1. Non compliant code

public IActionResult GetFile(string fileName)
{
    if (string.IsNullOrEmpty(fileName))
    {
        return NotFound();
    }
    
    ...

The above code is a simple file retrieval method in an ASP.NET Core Controller. This method takes a filename as a parameter and attempts to open and return this file. The filename is used directly to create a file path which is then opened. The vulnerability lies in the lack of validation of the `fileName` parameter. An attacker could provide a `fileName` that includes path traversal characters (like '../') to move up the directory structure and potentially access sensitive files outside of the intended directory. For example, if an attacker passed in `fileName` as `../../appsettings.json`, it would lead to the application returning the contents of the appsettings.json file which could contain sensitive application configuration data like database connection strings, API keys, etc. This is a classic example of a Path Traversal vulnerability, where an attacker can manipulate the file path to access files that should not be accessible.

2. Steps

• Implement input validation to ensure that the 'fileName' parameter is a valid file name.

• Use a whitelist approach to validate the 'fileName' parameter, allowing only specific characters or patterns.

• Avoid using user-provided input directly in constructing file paths.

• Consider using a secure file storage mechanism that does not rely on user-provided file names.

• Implement proper error handling and return appropriate error responses when a file is not found or an error occurs.

3. Secure code example

public IActionResult GetFile(string fileName)
{
    if (string.IsNullOrEmpty(fileName) || !IsValidFileName(fileName))
    {
        return NotFound();
    }
    
    ...

The updated code includes a new method `IsValidFileName(string fileName)`. This method uses a regular expression to validate the `fileName` parameter. The regular expression `^[a-zA-Z0-9_\\\\.-]+$` allows only alphanumeric characters, underscores, dots, and hyphens in the file name. This is a whitelist approach to input validation, which is generally more secure than a blacklist approach. The `GetFile(string fileName)` method now calls `IsValidFileName(string fileName)` to validate the `fileName` parameter. If the `fileName` parameter is null, empty, or invalid, the method returns a `NotFound` result. This approach prevents path traversal attacks by ensuring that the `fileName` parameter does not contain any special characters that could be used to navigate to other directories. It also avoids using user-provided input directly in constructing file paths, which is a common source of security vulnerabilities. In addition, the code continues to check if the file exists and returns a `NotFound` result if it does not. This provides proper error handling and appropriate error responses.

References

• 063. Lack of data validation - Path Traversal

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