Need

Implementation of robust data validation and sanitization mechanisms

Context

• Usage of C# for developing robust and efficient applications
• Usage of Microsoft.AspNetCore.Mvc for building web applications using the MVC architectural pattern
• Usage of System for low-level operations and platform-specific functionality

Description

Non compliant code

        using Microsoft.AspNetCore.Mvc;
using System;

namespace VulnerableApp.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Index(string message)
        {
            try
            {
                // Some logic here
            }
            catch (Exception ex)
            {
                return View("Error", message);
            }
        }
    }
}
        
        

The above code snippet represents a controller in an ASP.NET Core application. Inside this controller, there is an Index action which accepts a string parameter message.

This message parameter is directly reflected back to the user in case of an exception, without any form of validation or sanitization. This is where the vulnerability lies.

If an attacker modifies the message parameter to include malicious scripts, these scripts will be returned in the server response and executed by the victim's browser. This is a classic example of a Cross-Site Scripting (XSS) vulnerability, which can lead to a variety of attacks including cookie theft, session hijacking, defacement of websites, and more.

The lack of data validation in this context makes the application susceptible to such injection attacks. It's important to always validate and sanitize user input before reflecting it back to the user or using it in any other way.

Steps

• Implement input validation for the 'message' parameter in the 'Index' action.
• Use a whitelist approach to validate the input and only allow specific characters or patterns.
• Sanitize the input by removing or encoding any special characters that could be used for XSS attacks.
• Consider using built-in validation attributes or custom validation logic to ensure the input is safe.
• Handle any validation errors gracefully and provide appropriate error messages to the user.
• Avoid displaying the raw user input in error messages or any other output to prevent potential XSS vulnerabilities.

Compliant code

        using Microsoft.AspNetCore.Mvc;
using System;
using System.Text.RegularExpressions;

namespace VulnerableApp.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Index(string message)
        {
            try
            {
                // Validate the input
                if (!IsValidInput(message))
                {
                    return View("Error", "Invalid input");
                }

                // Some logic here
            }
            catch (Exception ex)
            {
                return View("Error", "An error occurred");
            }
        }

        private bool IsValidInput(string input)
        {
            // Define a whitelist of allowed characters
            var regex = new Regex("^[a-zA-Z0-9 .,!?]*$");

            // Check if the input matches the whitelist
            return regex.IsMatch(input);
        }
    }
}
        
        

The updated code includes a method IsValidInput that validates the message parameter. This method uses a regular expression to define a whitelist of allowed characters. The regular expression ^[a-zA-Z0-9 .,!?]*$ allows only alphanumeric characters, spaces, and a few special characters (., !, ?). If the message parameter contains any characters not in this whitelist, the method returns false, and the Index action returns an error view with a generic error message.

This approach prevents potential Cross-Site Scripting (XSS) attacks by ensuring that only safe characters are allowed in the message parameter. It also avoids displaying the raw user input in the error message, which could be exploited in an XSS attack.

The error messages are now generic and do not include any user input. This is a best practice for error handling, as it prevents potential information leakage and further reduces the risk of XSS attacks.

Remember, this is a basic example of input validation. Depending on your application's requirements, you might need to allow more characters or use a different validation strategy. Always test your validation logic thoroughly to ensure it works as expected and doesn't introduce new vulnerabilities.

References

• 192. Lack of data validation - Reflected Parameters