tldr;

You can use Fallback Policies in ASP.NET Core 3.0+ to require an Authenticated User by default. Conceptually, you can think of this as adding an [Authorize] attribute by default to every single Controller and Razor Page ONLY WHEN no other attribute is specified on a Controller or Razor Page (like [AllowAnonymous] or [Authorize(PolicyName="PolicyName")]).  See lines 9-11 below.

https://gist.github.com/scottsauber/06950b52ca45ab0b937dc59cad19e9de#file-startup-cs-L9-L11

A Quick Lap Around the [Authorize] and [AllowAnonymous] Attributes

In ASP.NET Core (and even previously in ASP.NET), we’ve had the ability to add a [Authorize] attribute to a resource (such as a Controller or Razor Page)…

This article shows how MFA could be forced on users to access sensitive pages within an ASP.NET Core Identity application. This could be useful for applications where different levels of access exist for the different identities. For example, users might be able to view the profile data using a password login, but an administrator would be required to use MFA to access the admin pages.

Code: https://github.com/damienbod/AspNetCoreHybridFlowWithApi

Blogs in this series

• Force ASP.NET Core OpenID Connect client to require MFA
• Send MFA signin requirement to OpenID Connect server using ASP.NET Core Identity and IdentityServer4
• Requiring MFA for Admin Pages in an ASP.NET Core Identity application

Extending the Login with a MFA claim

The application is setup using ASP.NET Core with Identity and Razor Pages. In this demo, the SQL Server was replaced with SQLite, and the nuget packages were updated. The AddIdentity method is used instead of AddDefaultIdentity one, so…