What if you log in to a site and the site owner knows whether you are simultaneously logged into Gmail, Facebook, Twitter or Digg?

The Register reported that a web-developer from Nottingham, UK, Mike Cardwell, knows just how to do it. The method devised uses status codes which are different when a user is logged in or not. Cardwell has written a Java script code containing the link of the site that he wants to know a user is logged into, like Facebook or Digg, which is embedded in his site. Thus when a user visits his site and simultaneously logs into Facebook, Twitter or Digg, Cardwell can tell that the user is also browsing these sites.

The code currently works with Firefox, Safari and Chrome. Thus when a user's browser interacts with Cardwell's code it returns the HTTP status code.

The scope of this discovery currently seems harmless until it starts tracking user movement across sites like porn, political interests site etc.