detect if the requests come from Office process

OfficeDetection.cs

/*
    Workaround below to detect Office request comes from the user smichtch in
    https://github.com/aspnet/AspNetKatana/issues/78
*/

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Globalization;
using System.Linq;
using System.Web;
using Owin;
using Microsoft.Owin.Security;
using Microsoft.Owin.Security.Cookies;
using Microsoft.Owin.Security.OpenIdConnect;
using Microsoft.Owin;
using System.Threading.Tasks;
using System.Net.Http;
using System.Text.RegularExpressions;
using System.Net;

        public void ConfigureAuth(IAppBuilder app)
        {
            app.SetDefaultSignInAsAuthenticationType(CookieAuthenticationDefaults.AuthenticationType);
            app.UseCookieAuthentication(new CookieAuthenticationOptions());
            app.Use<MsOfficeLinkPrefetchMiddleware>();
            app.UseOpenIdConnectAuthentication(
                new OpenIdConnectAuthenticationOptions
                {
                    ClientId = clientId,
                    Authority = authority,
                    PostLogoutRedirectUri = postLogoutRedirectUri,
                    // RedirectUri = "https://bbb.hoangbac.com"
                    RedirectUri = "https://testnonce.azurewebsites.net"
                });

            Microsoft.IdentityModel.Logging.IdentityModelEventSource.ShowPII = true;
        }

        public class MsOfficeLinkPrefetchMiddleware : OwinMiddleware
        {
            public MsOfficeLinkPrefetchMiddleware(OwinMiddleware next) : base(next) { }
            public override Task Invoke(IOwinContext context)
            {
                if (Is(context, HttpMethod.Get, HttpMethod.Head) && IsMsOffice(context))
                {
                    // Mitigate by preempting auth challenges to MS Office apps' preflight requests and
                    // let the real browser start at the original URL and handle all redirects and cookies.

                    // Success response indicates to Office that the link is OK.
                    context.Response.StatusCode = (int)HttpStatusCode.OK;
                    context.Response.Headers["Cache-Control"] = "no-cache, no-store, must-revalidate";
                    context.Response.Headers["Pragma"] = "no-cache";
                    context.Response.Headers["Expires"] = "0";
                }
                else if (Next != null)
                {
                    return Next.Invoke(context);
                }
                return Task.CompletedTask;
            }

            private static bool Is(IOwinContext context, params HttpMethod[] methods)
            {
                var requestMethod = context.Request.Method;
                return methods.Any(method => StringComparer.OrdinalIgnoreCase.Equals(requestMethod, method.Method));
            }

            private static readonly Regex _msOfficeUserAgent = new Regex(
                @"(^Microsoft Office\b)|([\(;]\s*ms-office\s*[;\)])",
                RegexOptions.CultureInvariant | RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.Compiled);

            private static bool IsMsOffice(IOwinContext context)
            {
                var headers = context.Request.Headers;
                var userAgent = headers["User-Agent"] ?? string.Empty;
                return _msOfficeUserAgent.IsMatch(userAgent)
                    || !string.IsNullOrWhiteSpace(headers["X-Office-Major-Version"]);
            }
        }