I have Facebook open in one tab, and I open YouTube in another. YouTube asks me if I want to sign in with my Facebook account. Can YouTube tell if I have Facebook open in a different tab or window?

  • Do you mean "can any webapps tell if I am logged into facebook?" evan if facebook is not open in another tab. Jul 21, 2010 at 9:32

6 Answers 6


Yes, it is possible to tell that you are logged in (or at least used the service). In at least two ways:

1) Explicit connection. If YouTube uses the Facebook API to connect and check whether Facebook knows who you are in that session.

2) Using CSS tricks, where a website can check whether you have specific (well-known) images or files in your browser cache. They cannot tell who you are, but they can figure out which services you use.

The second way is how bookmarking services or OpenID providers can show you your top five services out of several hundreds they support.


No, individual tabs and windows have no way of knowing which other tabs / windows are open.

  • This would be a real privacy concern if this were possible. Sites could track your presence on other sites without your knowing.
    – spoulson
    Jul 16, 2010 at 13:35
  • Sorry to break your 666 rep :P
    – Matchu
    Jul 17, 2010 at 3:34
  • No way, is not really correct. There are a couple of ways that sites can tell what other sites you have been to or currently have open sessions with. Pages can tell where you came from via referrer links, so if you are coming from Facebook to YouTube they could suggest you login using facebook based on the fact that you came from there. Also because facebook uses OAuth 2.0, YouTube can use it's OAuth protocol to tell that you are in a facebook OAuth session and suggest that you login using that same session.
    – drye
    Jul 19, 2010 at 12:15
  • This is "in theory". Things like flash and browser exploits make this less true in reality. Jul 20, 2010 at 4:30
  • A site can't directly access your history, but it can tell whether you've previously visited certain specific pages. scienceblogs.com/pharyngula/2010/12/…. (It can't, by this method, tell whether you were logged in.)
    – TRiG
    Dec 4, 2010 at 20:18

However, I do believe that OpenID and OAuth sites can detect if you are signed in to another OpenID or OAuth provider. This is how they know, and are trying to provide a service letting you know you can use that login on their site.


Another way sites could do this is based on a referrer link, so if you come to YouTube from Facebook, YouTube can detect this.


Yes, it is possible, using an invisible 1x1 GIF web bug to access your Facebook cookies.

This was the source of the infamous Facebook Beacon fiasco. As a result of the ensuing lawsuit, Facebook has ceased to do this, and most likely will not try to again.


It appears that Facebook DOES know you are logged-in on a number of websites - that's probably because you are seeing the new "Like" social widgets ("plugins") that are all over the web.

To the average user, it appears that FB knows you are logged in, because you see which of your friends have "liked" that site or piece of content. However, technically, the FB social plugins are small iframes that are still hosted on facebook.com:


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.