Abusing HTTP Status Codes to Expose Private Information

When you visit my website, I can automatically and silently determine if you're logged into Facebook, Twitter, GMail and Digg. There are almost certainly thousands of other sites with this issue too, but I picked a few vulnerable well known ones to get your attention. You may not care that I can tell you're logged into GMail, but would you care if I could tell you're logged into one or more porn