There's another trick you can use, for detecting if somebody is logged in to certain sites. See: https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Priva

There's another trick you can use, for detecting if somebody is logged in to certain sites. See: https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Private_Information https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Pri... Although, the Google test on that page is currently broken. The Facebook and Twitter ones aren't.