Demasking Google Users With a Timing Attack

An attacker could share a document with one or more email addresses, but uncheck the option that causes Google to send a notification. Now the attacking site can figure out when someone logged into any of the shared addresses visits their site. This is mostly useful for very targeted attacks, where an attacking site needs to behave differently based on who is viewing. This could be used for spear phishing, identification of government officials, demasking users of TOR, industrial mischief, etc.