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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?

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Do you mean "can any webapps tell if I am logged into facebook?" evan if facebook is not open in another tab. –  DoNotInstall Jul 21 '10 at 9:32
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6 Answers

up vote 3 down vote accepted

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.

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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:

http://developers.facebook.com/plugins

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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.

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

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It is possible to block your browser from transmitting referrer links if you so choose. You can block this in most browsers, the steps for firefox can be found here: cafe.elharo.com/privacy/… For Chrome, here is an extension: chrome.google.com/extensions/detail/… –  drye Jul 16 '10 at 16:37
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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.

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

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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 '10 at 13:35
    
Sorry to break your 666 rep :P –  Matchu Jul 17 '10 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 '10 at 12:15
    
This is "in theory". Things like flash and browser exploits make this less true in reality. –  Kendall Hopkins Jul 20 '10 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 '10 at 20:18
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