Prevent Cross-Site Scripting (XSS) in ASP.NET Core

Prevent Cross-Site Scripting (XSS) in ASP.NET Core

Accessing encoders in code

The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible.

To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. For example;

C#

public class HomeController : Controller
   {
       HtmlEncoder _htmlEncoder;
       JavaScriptEncoder _javaScriptEncoder;
       UrlEncoder _urlEncoder;

       public HomeController(HtmlEncoder htmlEncoder,
                             JavaScriptEncoder javascriptEncoder,
                             UrlEncoder urlEncoder)
       {
           _htmlEncoder = htmlEncoder;
           _javaScriptEncoder = javascriptEncoder;
           _urlEncoder = urlEncoder;
       }
   }

Encoding URL Parameters

If you want to build a URL query string with untrusted input as a value use the UrlEncoder to encode the value. For example,

C#

var example = ""Quoted Value with spaces and &"";
   var encodedValue = _urlEncoder.Encode(example);

After encoding the encodedValue variable will contain %22Quoted%20Value%20with%20spaces%20and%20%26%22. Spaces, quotes, punctuation标点符号 and other unsafe characters will be percent encoded to their hexadecimal十六进制 value, for example a space character will become %20.

Warning

Don't use untrusted input as part of a URL path. Always pass untrusted input as a query string value.

[Test]
        public void XssTest()
        {

            string input = ""Quoted Value with spaces and &"";
            WriteConsole($"BeforeEncode:{Environment.NewLine}{input}");

            var output = UrlEncoder.Default.Encode(input);
            WriteConsole($"UrlEncoder.Encode:{Environment.NewLine}{output}");

            output = JavaScriptEncoder.Default.Encode(input);
            WriteConsole($"JavaScriptEncoder.Encode:{Environment.NewLine}{output}");

            output = HtmlEncoder.Default.Encode(input);
            WriteConsole($"HtmlEncoder.Encode:{Environment.NewLine}{output}");

            output = HttpUtility.HtmlEncode(input);
            WriteConsole($"HttpUtility.HtmlEncode:{Environment.NewLine}{output}");

            output = AntiXssEncoder.HtmlEncode(input,false);
            WriteConsole($"AntiXssEncoder.HtmlEncode:{Environment.NewLine}{output}");
        }

        private void WriteConsole(string str)
        {
            Console.WriteLine(str);
            Console.WriteLine();
        }

BeforeEncode:
"Quoted Value with spaces and &"

UrlEncoder.Encode:
%22Quoted%20Value%20with%20spaces%20and%20%26%22

JavaScriptEncoder.Encode:
u0022Quoted Value with spaces and u0026u0022

HtmlEncoder.Encode:
"Quoted Value with spaces and &"

HttpUtility.HtmlEncode:
"Quoted Value with spaces and &"

AntiXssEncoder.HtmlEncode:
"Quoted Value with spaces and &"