How to find out Vkontakte id without authorization of a person?

Question

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Tell me how to identify a person who is not authorized anywhere on the site and get his VK ID on your site?
There are services that can do this like socfish.ru. What's the secret? How is this possible?

Answer 1

It's just that clickjacking isn't really possible right now. Means it what that open methods.

Answer 2

Sociate? Works norms, but devours the first click.
Works by simply adding an invisible iframe with authorization in the application that follows the mouse. There was already an article on Habré, maybe not even one.