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Lack of data validation - Host Header Injection

Need​

Implementation of strict input validation and sanitization to prevent host header injection

Context​

  • Usage of C# 7.0 for modern language features and enhancements
  • Usage of Microsoft.AspNetCore.Mvc for building web applications using the ASP.NET Core MVC framework
  • Usage of Microsoft.AspNetCore.Http for handling HTTP requests and responses in ASP.NET Core applications

Description​

Non compliant code​

// This is a sample vulnerable code in an ASP.NET Core application
public IActionResult Index()
{
    var host = Request.Headers["Host"].ToString();
    var redirectUrl = $"http://{host}/home";
    
    return Redirect(redirectUrl);
}

The above code is a simple controller action in an ASP.NET Core application.

The action is trying to redirect the user to the home page of the application. To construct the URL for redirection, it is using the "Host" header value from the incoming HTTP request.

The vulnerability here is that the code doesn't validate the "Host" header value before using it. An attacker can manipulate the "Host" header value to any arbitrary value.

For example, an attacker can set the "Host" header value to a malicious website. As a result, the application will redirect the user to this malicious website. This is known as Host Header Injection vulnerability.

This vulnerability can lead to various attacks like password reset poisoning, web-cache poisoning, and potentially to SSRF (Server-Side Request Forgery) attacks.

Steps​

  • Implement strict input validation for the 'Host' header.
  • Ensure that the 'Host' header only contains valid and expected values.
  • Use a whitelist approach to validate the 'Host' header against a predefined list of allowed hosts.
  • Sanitize the 'Host' header input to remove any potentially malicious characters or payloads.
  • Consider using a secure redirect mechanism that does not rely solely on the 'Host' header for redirection.
  • Regularly update and patch the application to address any new vulnerabilities.

Compliant code​

// This is a sample fixed code in an ASP.NET Core application
public IActionResult Index()
{
    var host = Request.Headers["Host"].ToString();
    var allowedHosts = new List<string> { "example.com", "example2.com" }; // Add your allowed hosts here

    if (!allowedHosts.Contains(host))
    {
        return BadRequest("Invalid host");
    }

    var redirectUrl = $"http://{host}/home";
    
    return Redirect(secureRedirectUrl);
}

The above code fixes the Host Header Injection vulnerability by implementing strict input validation for the 'Host' header.

Firstly, it defines a list of allowed hosts. This is a whitelist approach where only the hosts that are defined in the list are considered valid.

Then, it checks if the host from the request header is in the list of allowed hosts. If it is not, it returns a bad request response. This ensures that only valid and expected values are accepted for the 'Host' header.

Finally, it constructs the redirect URL and redirects to it. This secure redirect mechanism does not rely solely on the 'Host' header for redirection, thus preventing any potential Host Header Injection attacks.

Remember to regularly update and patch your application to address any new vulnerabilities.

References​

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