Skip to main content

DOM-Based cross-site scripting (XSS)

Need​

Prevention of cross-site scripting attacks

Context​

  • Usage of C# for developing robust and efficient software applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core MVC
  • Usage of System for managing and interacting with the operating system

Description​

Non compliant code​

using Microsoft.AspNetCore.Mvc;
using System;

namespace VulnerableApp.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Index(string userInput)
        {
            ViewData["Message"] = userInput;
            return View();
        }
    }
}

The above code is an example of a DOM-Based Cross-Site Scripting (XSS) vulnerability in a C# ASP.NET Core application. Here's why:

  1. The HomeController has an Index action method that accepts a string parameter userInput. This method assigns the userInput value to ViewData["Message"] without any form of validation or encoding.

  2. In the corresponding view Index.cshtml, the userInput value is directly output to the web page using @ViewData["Message"].

This means that if a user can control the userInput value, they can inject arbitrary HTML or JavaScript code into the web page, which is then executed by the browser. This is a serious security vulnerability, as it can lead to various attacks such as stealing sensitive information or performing actions on behalf of the user.

For example, an attacker could craft a URL like http://vulnerableapp.com/home/index?userInput=<script>/*malicious code*/</script> and trick a user into clicking it. The malicious script would then be executed in the user's browser when the page is loaded.

Steps​

  • Implement input validation to ensure that user input is safe and does not contain any malicious code.
  • Use output encoding to properly sanitize user input before displaying it in the web application.
  • Avoid generating dynamic web content from the client side. Instead, generate it server-side and properly validate and sanitize the data before rendering it.
  • Implement a Content Security Policy (CSP) to restrict the sources from which the web application can load content.
  • Regularly update and patch the ASP.NET Core framework and any third-party libraries used in the application to ensure that any known vulnerabilities are addressed.

Compliant code​

using Microsoft.AspNetCore.Mvc;
using System;
using System.Text.Encodings.Web;

namespace VulnerableApp.Controllers
{
    public class HomeController : Controller
    {
        private readonly HtmlEncoder _htmlEncoder;

        public HomeController(HtmlEncoder htmlEncoder)
        {
            _htmlEncoder = htmlEncoder;
        }

        public IActionResult Index(string userInput)
        {
            string encodedInput = _htmlEncoder.Encode(userInput);
            ViewData["Message"] = encodedInput;
            return View();
        }
    }
}

The original code was vulnerable to DOM-Based Cross-Site Scripting (XSS) attacks because it directly used user input to generate dynamic web content without validating or sanitizing the input. This could allow an attacker to inject malicious scripts into the web application, which would then be executed in the user's browser.

The fixed code addresses this vulnerability by implementing input validation and output encoding. Specifically, it uses the HtmlEncoder.Encode method from the System.Text.Encodings.Web namespace to sanitize the user input before using it to generate dynamic web content. This method encodes special characters in the input, effectively neutralizing any potentially malicious scripts.

In addition to this code fix, it is recommended to implement a Content Security Policy (CSP) to restrict the sources from which the web application can load content. This can further mitigate the risk of XSS attacks by preventing the execution of unauthorized scripts.

Finally, it is important to regularly update and patch the ASP.NET Core framework and any third-party libraries used in the application. This ensures that any known vulnerabilities are addressed and helps maintain the overall security of the application.

References​

Last updated on