How do services work that determine the VKontakte id of site visitors?

Question

Computer networks

There are a number of services that allow you to get the VKontakte id of users visiting the site (for example, soceffect or soctracker).
How it works? Is the standard VKontakte API used? Does he allow this? Or are some vulnerabilities exploited?
I found examples of sites using such services: 55555555.ru and ucvvm.ru
When the page is loaded, a get-request is executed that sends the time the user entered the site page, some unique identifier, and something else. In response, an obfuscated code comes, which, apparently, determines the id of the VK user. I am far from web development, so I don’t have enough skills for a more complex analysis.

Answer 1

Google clickjacking, everything is described in detail.

Answer 2

Is the standard VKontakte API used? Does he allow this?

Yes.