I am trying to figure out how the different authentication types in SharePoint online work. I am doing this research because I want check the impact of disabling legacy authentication in my tenant. This is a follow up on my question Question about SharePoint online credentials

Based on this image

SharePoint online authentication types

As far as I know these are the types we currently use.

When logging in from the browser:

  1. ADFS (on-prem accounts)
  2. Cloud Authentication using Cloud only account -> Azure AD as IDP

When logging in from code

  1. From code using SharePoint online credentials
  2. Using an Azure AD App registration

When I use ADFS I use federated authentication. I see that 2 cookies are set by SharePoint. These cookies are named "FedAuth" and "rtFa".

Question 1: When legacy authentication is disabled, cookie based authentication is disabled. So will ADFS still work?

Question 2: When I use cloud only accounts, am I still using federated authentication? is Azure AD used as the ADFS server? Or am I authenticated against Azure AD using other authentication type? Because in this situation I also see these cookies "FedAuth" and "rtFa" being set.

Question 3 When I connect from code using CSOM

using (ClientContext clientContext = new ClientContext("https://yoursite.sharepoint.com/"))  
    SecureString passWord = new SecureString();
    clientContext.Credentials = new SharePointOnlineCredentials("loginname@yoursite.onmicrosoft.com", passWord);
    Web web = clientContext.Web;

I see this authentication flow

Authentication flow

A cookie named "SPOIDCRL" is created and used as authentication (I can use this in Postman for example) What kind of authentication is this? A SAML token is send to the srf endpoint and I am authenticated using the idcrl.svc endopint.

This authentication type (SharePoint online credentials) stops working when I disable legacy authentication.

Using the Azure AD app registration is clear to me.

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