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Reflected cross-site scripting (XSS)

Need​

Implementation of input validation and output encoding to prevent XSS attacks.

Context​

  • Usage of C# for developing robust and efficient applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core
  • Usage of the System package for accessing and manipulating system-level resources and functionality.

Description​

Non compliant code​

using Microsoft.AspNetCore.Mvc;
using System;

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

In the code snippet above, the HomeController class contains an Index method that takes a name parameter from the HTTP request and directly uses it to generate a greeting message. This message is then passed to the view via ViewData.

This piece of code is vulnerable to Reflected Cross-Site Scripting (XSS) attacks. If an attacker injects malicious script as the name parameter in the URL, the script will be executed on the client's browser when the page is loaded. This is because the application does not validate or encode the name parameter before using it to generate the dynamic content.

For example, an attacker could craft a URL like http://vulnerableapp.com/home/index?name=<script>malicious_code_here</script>. When a victim visits this URL, the malicious script is executed in their browser, potentially leading to theft of sensitive data, session hijacking, or other malicious actions.

Steps​

  • Implement input validation to ensure that the 'name' parameter does not contain any malicious code.
  • Use output encoding to sanitize the 'name' parameter before displaying it in the view.
  • Consider using a content security policy (CSP) to restrict the types of content that can be loaded by the web application.
  • Regularly update and patch the ASP.NET Core framework to benefit from the latest security enhancements.

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 name)
        {
            string encodedName = _htmlEncoder.Encode(name);
            ViewData["Message"] = "Hello " + encodedName;
            return View();
        }
    }
}

The original code was vulnerable to a reflected cross-site scripting (XSS) attack because it was dynamically generating web content without validating the source of the potentially untrusted data. In this case, the 'name' parameter was being directly inserted into the ViewData dictionary without any sanitization or encoding.

The fixed code mitigates this vulnerability by using the HtmlEncoder class to sanitize the 'name' parameter before it is inserted into the ViewData dictionary. The HtmlEncoder.Encode method is used to encode the 'name' parameter, which effectively sanitizes it by converting potentially harmful characters into their safe equivalents. This prevents any malicious scripts from being executed when the 'name' parameter is displayed in the view.

In addition to this, it is recommended to implement a content security policy (CSP) to restrict the types of content that can be loaded by the web application. This can further protect against XSS attacks by preventing the loading of malicious scripts.

Finally, it is important to regularly update and patch the ASP.NET Core framework to benefit from the latest security enhancements. This can help to protect against new vulnerabilities that may be discovered in the future.

References​

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