How does Yula calculate my VK via vk connect?

Question

HTTP Cookies

As you can see in the picture, Yula knows my VK, how did they know my VK? I know that vk and yula are both the brainchild of mail ru, but it seems that Cookie is set only for a specific domain, how does it work? I would also like to have this on my site, so that later I can do clickjacking that recognizes not only VK but also the number.

Answer 1

From Yula, a request is made to the mail.ru server, and authorization cookies are most likely stored on the mail authorization domain

Answer 2

It's like one company, so it's easy. For example youtube and google.
with yt there is a request to accounts google com with the returnurl youtube com parameter and with a key, only this redirect is fast. So make yourself a separate domain like login.example.com and carry out all authorization only through it, passing parameters via GET, in tokens.
And about VK:
https://habr.com/ru/post/228617/
Is it possible to identify a site visitor's VK?
Many can be found.