Program.cs

using Microsoft.AspNetCore.Authentication.OpenIdConnect;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc.Authorization;
using Microsoft.Identity.Web;
using Microsoft.Identity.Web.UI;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddSession(options =>
 {
   options.Cookie.IsEssential = true;
 });

var initialScopes = builder.Configuration.GetSection("DownstreamApi:Scopes")
    .Value
    .Split(' ');

builder.Services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApp(builder.Configuration)
    .EnableTokenAcquisitionToCallDownstreamApi(initialScopes)
    .AddDistributedTokenCaches();

builder.Services.AddControllersWithViews(options =>
{
    var policy = new AuthorizationPolicyBuilder()
        .RequireAuthenticatedUser()
        .Build();

    options.Filters.Add(new AuthorizeFilter(policy));
}).AddMicrosoftIdentityUI();

builder.Services.AddRazorPages();

var app = builder.Build();

app.UseSession();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

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

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(
        name: "default",
        pattern: "{controller=Home}/{action=Index}/{id?}");
    endpoints.MapRazorPages();
});

app.Run();