Azure AD Multi Tenant ,.Net Core Web API with MSAL(Microsoft Authentication Libary)

Question

I believe I have the Microsoft Authentication Library (MSAL) JavaScript pulling back a JWT token, using azure AD multi tenant with the following config. Based of this link https://docs.microsoft.com/en-us/azure/active-directory/develop/howto-convert-app-to-be-multi-tenant . I believe I only need the following two values.

clientId: "A134d6c8-8078-2924-9e90-98cef862eb9a" // this would be the app registrations client id(application)
authority: "https://login.microsoftonline.com/common"

How then can I configure a .net core 3 web api that can handle this JWT token and authenticate [Authorize] endpoints by me passing the Authorization: Bearer header.

I currently get this error in the response which is not very helpful!

AuthenticationFailed: IDX10511: Signature validation failed. Keys tried: '[PII is hidden. For more details, see https://aka.ms/IdentityModel/PII.]'. 
kid: '[PII is hidden. For more details, see https://aka.ms/IdentityModel/PII.]'. 
Exceptions caught:
 '[PII is hidden. For more details, see https://aka.ms/IdentityModel/PII.]'.
token: '[PII is hidden. For more details, see https://aka.ms/IdentityModel/PII.]'.

The Startup.cs code is as follows

using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.IdentityModel.Tokens;

namespace MultiTenantApi
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {

            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors(x =>
            {
                x.AddDefaultPolicy(cfg =>
                {
                    cfg.AllowAnyOrigin()
                        .AllowAnyHeader()
                        .AllowAnyMethod();
                });
            });

            services.AddAuthentication(cfg =>
                {
                    cfg.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                    cfg.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
                })
                .AddJwtBearer(opt =>
                {
                    opt.Authority = "https://login.microsoftonline.com/common";
                    opt.Audience = "api://A134d6c8-8078-2924-9e90-98cef862eb9a"; // Set this to the App ID URL for the web API, which you created when you registered the web API with Azure AD.
                    opt.TokenValidationParameters = new TokenValidationParameters
                    {
                        ValidateIssuer = false
                    };
                    opt.Events = new JwtBearerEvents()
                    {
                        OnAuthenticationFailed = AuthenticationFailed
                    };
                });

            services.AddControllers();

        }

        private Task AuthenticationFailed(AuthenticationFailedContext arg)
        {
            // For debugging purposes only!
            var s = $"AuthenticationFailed: {arg.Exception.Message}";
            arg.Response.ContentLength = s.Length;
            arg.Response.Body.WriteAsync(Encoding.UTF8.GetBytes(s), 0, s.Length);
            return Task.FromResult(0);
        }
        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            app.UseStaticFiles(); // Added

            app.UseRouting();
            app.UseCors(); //Added

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });

        }

    }
}

Answer 1

In your ConfigureServices methods, add IdentityModelEventSource.ShowPII = true; to show detail of error and see the problem.

Reference:

1. asp.net azure active directory integration error message contains '[PII is hidden]'
2. PII is hidden error #51