How can I implement the Anti-Forgery Token between MVC frontend and Web Api on different domains?

Question

1. I want to have the MVC project on www.example1.com
2. WebApi project on api.example2.com

I want to restrict the access to WebApi. I've tried to implement the Anti-Forgery Token:

When I create the GET request to WebApi with Anti-forgery token then I get an exception because the request doesn't contains this token.

In method called ValidateRequestHeader is variable cookie = null.

How can I fix following code? Is this correct solution?

MVC project (front-end) - for development is localhost:33635:

Index.cshtml

<div class="container">


    <div class="row">

        <div class="col-md-12">


            <input id="get-request-button" type="button" class="btn btn-info" value="Create request to API Server" />

            <br />

            <div id="result"></div>

        </div>


    </div>


</div>


@section scripts
{

    <script type="text/javascript">

        @functions{
            public string TokenHeaderValue()
            {
                string cookieToken, formToken;
                AntiForgery.GetTokens(null, out cookieToken, out formToken);
                return cookieToken + ":" + formToken;
            }
        }

        $(function () {

            $("#get-request-button").click(function () {

                $.ajax("http://localhost:33887/api/values", {
                    type: "GET",
                    contentType: "application/json",
                    data: {},
                    dataType: "json",
                    headers: {
                        'RequestVerificationToken': '@TokenHeaderValue()'
                    }
                }).done(function (data) {
                    $("#result").html(data);
                });

                return false;
            });

        });


    </script>

}

WebApi project - for development is localhost:33887:

WebApiConfig.cs

public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            config.EnableCors(new EnableCorsAttribute("http://localhost:33635", "*", "*"));

            // Web API routes
            config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                    name: "DefaultApi",
                    routeTemplate: "api/{controller}/{id}",
                    defaults: new { id = RouteParameter.Optional }
            );
        }

ValidateHttpAntiForgeryTokenAttribute.cs:

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
    public sealed class ValidateHttpAntiForgeryTokenAttribute : FilterAttribute, IAuthorizationFilter
    {
        public Task<HttpResponseMessage> ExecuteAuthorizationFilterAsync(HttpActionContext actionContext, CancellationToken cancellationToken, Func<Task<HttpResponseMessage>> continuation)
        {
            var request = actionContext.Request;

            try
            {
                if (IsAjaxRequest(request))
                {
                    ValidateRequestHeader(request);
                }
                else
                {
                    AntiForgery.Validate();
                }
            }
            catch (Exception)
            {
                actionContext.Response = new HttpResponseMessage
                {
                    StatusCode = HttpStatusCode.Forbidden,
                    RequestMessage = actionContext.ControllerContext.Request
                };
                return FromResult(actionContext.Response);
            }
            return continuation();
        }

        private Task<HttpResponseMessage> FromResult(HttpResponseMessage result)
        {
            var source = new TaskCompletionSource<HttpResponseMessage>();
            source.SetResult(result);
            return source.Task;
        }

        private bool IsAjaxRequest(HttpRequestMessage request)
        {
            IEnumerable<string> xRequestedWithHeaders;
            if (!request.Headers.TryGetValues("X-Requested-With", out xRequestedWithHeaders)) return false;

            var headerValue = xRequestedWithHeaders.FirstOrDefault();

            return !String.IsNullOrEmpty(headerValue) && String.Equals(headerValue, "XMLHttpRequest", StringComparison.OrdinalIgnoreCase);
        }

        private void ValidateRequestHeader(HttpRequestMessage request)
        {
            var headers = request.Headers;
            var cookie = headers
                    .GetCookies()
                    .Select(c => c[AntiForgeryConfig.CookieName])
                    .FirstOrDefault();

            IEnumerable<string> xXsrfHeaders;

            if (headers.TryGetValues("RequestVerificationToken", out xXsrfHeaders))
            {
                var rvt = xXsrfHeaders.FirstOrDefault();

                if (cookie == null)
                {
                    throw new InvalidOperationException($"Missing {AntiForgeryConfig.CookieName} cookie");
                }

                AntiForgery.Validate(cookie.Value, rvt);
            }
            else
            {
                var headerBuilder = new StringBuilder();

                headerBuilder.AppendLine("Missing X-XSRF-Token HTTP header:");

                foreach (var header in headers)
                {
                    headerBuilder.AppendFormat("- [{0}] = {1}", header.Key, header.Value);
                    headerBuilder.AppendLine();
                }

                throw new InvalidOperationException(headerBuilder.ToString());
            }
        }
    }

ValuesController:

public class ValuesController : ApiController
    {
        // GET: api/Values
        [ValidateHttpAntiForgeryToken]
        public IEnumerable<string> Get()
        {
            return new string[] { "value1", "value2" };
        }

        // GET: api/Values/5
        public string Get(int id)
        {
            return "value";
        }

        // POST: api/Values
        public void Post([FromBody]string value)
        {
        }

        // PUT: api/Values/5
        public void Put(int id, [FromBody]string value)
        {
        }

        // DELETE: api/Values/5
        public void Delete(int id)
        {
        }
    }

Answer 1

This is not the way to restrict the access by another service. The ForgeryToken helps to prevent CSRF attacks, ASP.NET MVC uses anti-forgery tokens, also called request verification tokens. The client requests an HTML page that contains a form. The server includes two tokens in the response. One token is sent as a cookie. When you submit the form, those will match on the same server.

I believe, What you need is the trust from example1.com to api.example2.com. An example would be stackauth which is a domain for centralized services across the entire Stack Exchange network. Then you can implement the authorization as you need in your project.