What the difference between these endpoint to authenticate RST2.srf vs extSTS.srf?

Question

We use two endpoints https://login.microsoftonline.com/extSTS.srf and https://login.microsoftonline.com/RST2.srf to authenticate.

Can anybody explain to me the difference between them?

Answer 1

There are 2 flavors of authentication: one with a custom STS and one without (Using MSO STS only). The end goal is to retrieve the authentication cookie (SPOIDCRL cookie).

The one with custom STS is extSTS.srf, the other one without custom STS is RST2.srf.

Reference:

SharePoint Online Active Authentication