Possible to find out whether a user is logged into facebook over javascript API?

Question

This question is not a duplicate of this one.

I don\'t want to know whether the user has authorized my application, but if the user is logged into facebook (complete

Answer 1

This article

https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Private_Information

identifies security risks in Google and Facebook that will allow you to determine if a user is logged in. While no official API exists to check if a user is logged in without that user giving you express permission to access this information, the above article shows how you can 'guess' if a user is logged in or not.

Note: The article identifies a 'hack' and so is not guaranteed to work in the future, if or when Google & Facebook identify these security risks.