.NET How to NOT authenticate if provided user from OIDC is not available in database

So my .NET Core WebApplication is logging in an user via Azure AD and I have a database with users and their roles.

I created already OIDC MiddleWare to add Claims from my database for the user that tries to log in.

So the flow is:

• I login with my AD account
• I get the email address and check in the database for it's role
• Now if the user is blocked or not assigned, the login should fail

So my question: Is there a way how I could deny the authentication for the user, when he is not available in the DB or blocked?

What I did now is that I set a Claim and if that claim is not available it will be redirected to Access Denied page (by using the AuthorizationPolicy), but I want that the user will be redirected to the login page from Microsoft/AD (in best case with a message).

Is that possible somehow and if so, how ?

This is my code now:

 services.AddAuthentication(AzureADDefaults.AuthenticationScheme)
     .AddAzureAD(options => { Configuration.Bind("AzureAd", options); }
 );

 services.AddControllersWithViews(options =>
 {
   var policy = new AuthorizationPolicyBuilder()
                .RequireAuthenticatedUser()
                .RequireAssertion(context =>
                {
                    Claim claim = context.User.Claims.First(claim => claim.Type == ClaimTypes.Expired);
                    return context.User.HasClaim(x => x.Type == ClaimTypes.Expired) &&
                           context.User.Claims.First(claim => claim.Type == ClaimTypes.Expired).Value
                               .Equals("false", StringComparison.OrdinalIgnoreCase);
                })
                .RequireClaim(ClaimTypes.Name)
                .RequireClaim(ClaimTypes.Role)
                .Build();
            options.Filters.Add(new AuthorizeFilter(policy));
        });

 // OIDC Middleware, to access the User's Claims while logging in through AzureAD
        services.Configure<OpenIdConnectOptions>(AzureADDefaults.OpenIdScheme, options =>
        {
            options.Events = new OpenIdConnectEvents
            {
                OnRemoteFailure = ctx =>
                {
                    ctx.Response.Redirect("/");
                    ctx.HandleResponse();
                    return Task.CompletedTask;
                },
                OnSignedOutCallbackRedirect = ctx =>
                {
                    ctx.Response.Redirect("/");
                    ctx.HandleResponse();
                    return Task.CompletedTask;
                },
                OnTokenValidated = ctx =>
                {
                    // Get the user's email 
                    var email = ctx.Principal.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Name)?.Value;

                    // Query the database to get the role
                    using (var db = ctx.HttpContext.RequestServices.GetRequiredService<TracingContext>())
                    {
                        // Get the Users from the database, with the logged in email address (from Azure)
                        User user = db.Users.FirstOrDefault(u => u.UPN.Equals(email));

                        if (user != null)
                        {
                            user.LastLogin = DateTime.Now;
                            db.SaveChanges();
                            
                            // Add claims
                            var claims = new List<Claim>
                            {
                                new Claim(ClaimTypes.Role, user.Role.ToString()),
                                new Claim(ClaimTypes.Expired, (!user.IsActivated || user.IsBlocked).ToString())
                            };

                            // Save the claim
                            var appIdentity = new ClaimsIdentity(claims);
                            ctx.Principal.AddIdentity(appIdentity);
                        }
                        else
                        {
                           **// Send back to Login Page (with error message, maybe?)**
                        }
                    }
                    return Task.CompletedTask;
                },
            };
        });

You can override the default processing and handle the response yourself:

// Send back to Login Page (with error message, maybe?)
ctx.HandleResponse();
ctx.Response.Redirect("/path/to/login");

The call to HandleResponse signals that we want to handle the response ourselves and the following call sets up the redirect. There are a few approaches to sending over an error message. One approach is to provide a query-string parameter to the login URL.