Need

Secure functionality to prevent exploitation by attackers

Context

• Usage of C# for building robust and scalable applications
• Usage of System for accessing and manipulating system-level functionalities
• Usage of System.IO for Input/Output operations in .NET development
• Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core

Description

Non compliant code

        using System;
using System.IO;
using Microsoft.AspNetCore.Mvc;

namespace VulnerableApp.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class FileController : ControllerBase
    {
        [HttpGet("{fileName}")]
        public IActionResult Get(string fileName)
        {
            var path = Path.Combine(Directory.GetCurrentDirectory(), "Files", fileName);
            var file = System.IO.File.ReadAllText(path);
            return Ok(file);
        }
    }
}
        
        

The above code represents a simple ASP.NET Core controller with a single action that reads a file from the server and returns its content. This action is vulnerable to Path Traversal attacks as it does not validate or sanitize the fileName parameter.

An attacker can manipulate the fileName parameter to include ../ sequences, which can potentially allow them to read any file on the system that the web server has access to. For example, an attacker could request /api/file/../../../etc/passwd to retrieve a critical system file.

This is a severe security issue as it can lead to information disclosure, allowing an attacker to gain information about the system, its structure, and its files. This information can be used for further attacks.

Steps

• Implement proper access control mechanisms to restrict unauthorized access to sensitive files.
• Validate user input to prevent directory traversal attacks.
• Use a secure file storage mechanism to store sensitive files.
• Implement proper error handling to prevent information leakage.
• Consider implementing authentication and authorization mechanisms to control access to the file functionality.

Compliant code

        using System;
using System.IO;
using System.Linq;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;

namespace SecureApp.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    [Authorize] // Add authorization
    public class FileController : ControllerBase
    {
        private static readonly string[] ValidFileExtensions = new[] { ".txt", ".pdf" }; // Define valid file extensions

        [HttpGet("{fileName}")]
        public IActionResult Get(string fileName)
        {
            // Validate file extension
            var fileExtension = Path.GetExtension(fileName);
            if (!ValidFileExtensions.Contains(fileExtension))
            {
                return BadRequest("Invalid file extension.");
            }

            // Prevent directory traversal attacks
            if (fileName.Contains(".."))
            {
                return BadRequest("Invalid path.");
            }

            var path = Path.Combine(Directory.GetCurrentDirectory(), "Files", fileName);
            if (!System.IO.File.Exists(path))
            {
                return NotFound();
            }

            var file = System.IO.File.ReadAllText(path);
            return Ok(file);
        }
    }
}
        
        

The updated code includes several security improvements to address the insecure functionality vulnerability:

1. Access Control: The [Authorize] attribute is added to the FileController class. This ensures that only authenticated users can access the file functionality.

2. Input Validation: The code now checks the file extension against a list of valid extensions before processing the request. This helps to prevent the execution of potentially malicious files. Additionally, the code checks for directory traversal attacks by looking for ".." in the file name.

3. Secure File Storage: The files are stored in a directory within the current directory of the application. This is a simple form of secure file storage, but in a real-world application, you might want to consider more secure options such as storing files in a database or using a cloud storage service with strong access controls.

4. Error Handling: The code now returns appropriate HTTP status codes and error messages when something goes wrong, such as when the requested file does not exist or the file name is invalid. This helps to prevent information leakage.

5. Authentication and Authorization: The [Authorize] attribute also provides a basic form of authorization by restricting access to authenticated users. In a real-world application, you might want to implement more granular access controls based on user roles or permissions.

References

• 014. Insecure functionality