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YOUR SOCIAL MEDIA FINGERPRINT

Without your consent most major web platforms leak whether you are logged in.
This allows any website to detect on which platforms you're signed up. Since
there are lots of platforms with specific demographics an attacker could reason
about your personality, too.


DEMONSTRATION




YOU ARE LOGGED IN TO:


NO PLATFORM

(or maybe you've disabled third party cookies, or you are using something like
Privacy Badger?)









YOU ARE NOT LOGGED IN TO:

FacebookTumblrGmailSquareTwitterYoutubeSpotifyRedditGoogle
PlusBattle.netDropboxFoursquareAmazon.comBloggerSkypeCarbonmadeGithubAcademia.eduKhan
Academy500pxEdXSlackSteamExpediaDisqusMediumPaypalHackernewsIndeedVKMeetupBitBucketPinterestAirbnb




EXPLANATION

For most web platforms there's a way to abuse the login mechanism to detect
whether a user is logged in to that service.
Although this vulnerability is well known for several years most companies won't
fix it.

The exploit is pretty simple and actually easy to fix. Let's look at
facebook.com to get an idea of how it works:

HOW DOES THE LOGIN REDIRECT MECHANISM WORK?


At first we need to understand the login redirect mechanism. Say you're logged
in and visit



https://www.facebook.com/bookmarks/pages

Now if you open this URL in a private tab where you are not logged in, you will
get redirected to the login screen with this URL:

https://www.facebook.com/login.php?next=https%3A%2F%2Fwww.facebook.com%2Fbookmarks%2Fpages

Note the next parameter in the URL:
https%3A%2F%2Fwww.facebook.com%2Fbookmarks%2Fpages.

That's the URL we came from and it is used to redirect us back there once we've
logged in.
But if we enter the login URL in a browser tab where we are already logged in,
we get redirected without getting prompted to login. Therefore this URL returns:

 * If logged in:   the resource at the URL in the next parameter
 * If logged out: the login screen

BYPASS THE SAME ORIGIN POLICY

How does this URL help us? The Same Origin Policy prevents to read the results
of this request from any other domain but https://facebook.com...
Well, the SOP is strict for HTML pages, but it allows to receive images from
other origins! So if the resource in the next parameter would be an image we
could read it from our website. It can't be any image though. Facebook checks if
the URL in the next parameter starts with https://facebook.com. So we need to
find an image on facebook.com. Should be easy, right? Actually it isn't, because
facebook hosts almost all images on their CDN servers under the domain
fbcdn.net. Though there is one image that you can find on almost every
webserver: the good old favicon.ico!
This is how our login URL looks like with the favicon as next parameter:

https://www.facebook.com/login.php?next=https%3A%2F%2Fwww.facebook.com%2Ffavicon.ico

It has a very interesting property:
 * Logged in:   returns the favicon image
 * Logged out: returns the HTML of the login page

We can use this URL in an <img> tag in our website:

<img src="https://www.facebook.com/login.php?next=https%3A%2F%2Fwww.facebook.com%2Ffavicon.ico">

This <img> tag's property:
 * Logged in:   receives the favicon image, will load it successfully, and the
   onLoad callback will be fired.
 * Logged out: receives the HTML of the login screen, will fail to load it as an
   image, and the onError callback will be fired.

This leads to the final exploit:

<img onload="alert('logged in to fb')" onerror="alert('not logged in to fb')" src="https://www.facebook.com/login.php?next=https%3A%2F%2Fwww.facebook.com%2Ffavicon.ico">

OTHER PLATFORMS

This mechanism works for almost all major web platforms, because they all have a
redirect parameter in their login endpoint and they need to host their favicon
on their domain (bc of browser compatibility, and of the technical debt of the
former lack of standards).

UPDATES



 * 2016/10/07: Instagram removed the favicon from their root domain and host
   their favicon on their CDN now.
 * 2016/10/14: Stackoverflow has fixed the issue.
 * 2016/10/14: It looks like Firefox has fixed the issue at least for the major
   networks with their default tracking protection blacklist (The blacklist
   comes from https://disconnect.me/).
   
   Console screenshot of the errors when visting socialmedia-leak with current
   Firefox: Though it probably still works for all platforms where you don't see
   a broken image instead of a logo.
 * 2016/10/14: Netflix now just redirects to the main browse page regardless of
   the redirect in the URL.
 * 2017/11/29: Flickr has fixed the issue, too.






FURTHER ATTACKS

This attack could be a step of a worse attack such as deanonymization
techniques, Clickjacking, Profilejacking, or Phishing where it is crucial to
know if a victim is logged in to a service.


PROTECTION

First of all: disable third party cookies.

You can also install a browser plugin like Privacy Badger, or uMatrix to protect
yourself from this attack.

For further information see the discussion on Hackernews or this discussion on
Reddit.


HELP TO SOLVE THE ISSUE!

The companies won't fix the issue, because the risk for their own service is
relatively low. Though so many platforms are vulnerable to this, it becomes a
real privacy issue. Please help to raise awareness to push the companies to fix
the issue.

 * Share this demo on Twitter
 * Share this demo on Facebook
 * Share this demo on Google+
 * Share this demo via Whatsapp
 * Star this project on Github

If you find more web platforms that are vulnerable to this attack, please file
an issue to add it. The more complete this list is, the bigger the issue and the
higher the pressure for platforms to fix it.













IF YOU LIKE THIS PAGE MAYBE YOU LIKE MY OTHER WORKS, TOO:

 * Snapdrop: Instantly share files with devices nearby. No Setup, No Signup.
 * Webkay: A demonstration of all the personal information your browser is
   leaking.
 * Ubercookie: An irrevocable persistent browser cookie.
 * Web Security Quiz: Test your knowledge with OWASP exam questions
 * Capira: Interactive Learning Videos
 * Facebook: Like my page to get updates about the stuff I create
 * Twitter: Follow me on Twitter to get updates about the stuff I create

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