How to make client authorization "like Habrahabr"?

Question

Node.js

Good afternoon!
There was a need to move all authorization pages (from all our sites) to a separate site, as done by Habr with their https://id.tmtm.ru/login/ .
Actually the whole question is, how is it possible to implement it?
As well as related questions:
* How to check the user's session token on each site? Do I need to make a REST request to the authorization site to check the token, or does each site itself check the validity of the token and its session?
* Is it possible to fumble cookies not only on subdomains, but also on domains?
* Is there a ready-made plugin for NodeJS (ala express-session ) that allows you to check cross-domain session tokens, and generally fumble sessions between domains?

Answer 1

For personal searches :
=======================
*This method is called SSO

Answer 2

For example
https://habrahabr.ru/post/229375/