How to check the registration of a person on another site?

Question

User identification

Hi everybody!
There was a need to check if a person is registered on the site example.com.
GooglePlus, VK, FB, have their own APIs - this is understandable. But I need to check registration on a site without public API. The site saves a cookie during authorization, you can read the cookie, although I don’t know if it’s possible to read the cookies of another host.
Who has any ideas? maybe someone faced with a similar problem?
Thanks in advance!

Answer 1

No, it is not possible to read cookies from another host. If you think about it, this possibility would make the web hell.
On the site, which was made by a crooked phpshnik, diligently, as they like, writing out 55 conditions when checking a password, and displaying a separate error message for each, you can check in theory.
On a normal site - you can't, and that's great.