Vote here to get group memberships added to Azure AD B2C Claims: https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/10123836-get-user-membership-groups-in-the-claims-with-ad-b

Azure.B2C.Demos.GroupAuthorization

In this project, we demonstrate the ability to verify a user's existence against an Active Directory group from within an Azure Active Directory B2C tenant. To do this, we use a custom authentication service that implements the Microsoft.AspNetCore.Authorization.IAuthorizationService interface and uses the Azure AD Graph API to access and confirm our user is a member of the specified group.

This demo is very limited in scope, only showing how to build middleware to verify an [Authorize] call, but there's no stopping you (or anyone else) from taking things up a notch. 😎

This demo also uses the HttpClient-driven approach to development over using ActiveDirectoryClient for the simple fact that ActiveDirectoryClient isn't supported on Linux.

Startup.cs

public void ConfigureServices(IServiceCollection services)
{

	//register our custom authorization service
    services.AddSingleton<IAuthorizationService,GroupAuthService>();

	//some parts of our application require a user to be a member of the Employers AD group.
	services.AddAuthorization(options => options.AddPolicy("Employers",builder => builder.RequireRole("Employers")));

}

HomeController.cs

	[Authorize("Employers")]
	public IActionResult Auth()
	{
	    return View();
	}

For the nitty-gritty

All of the dirty work is done in the GroupAuthService authorization service.

Enjoy!