using System;

using System.Net;   

​

public class Program

{

    public static void Main()

    {

        var req = WebRequest.CreateHttp(new Uri("http://example.com"));

        try

        {

            req.Host = "example.com:@attacker.com";

            Console.WriteLine(req.Host);

        }

        catch(ArgumentException ex)

        {

            Console.WriteLine("Invalid host!");

        }

    }

}