A new kind of tracking tool, canvas fingerprinting, is being used to follow visitors to thousands of top websites, from WhiteHouse.gov to YouPorn. First documented in a forthcoming paper by researchers at Princeton University and KU Leuven University in Belgium, this type of tracking, called canvas fingerprinting, works by instructing the visitor’s Web browser to draw a hidden image. Because each computer draws the image slightly differently, the images can be used to assign each user’s device a number that uniquely identifies it. Like other tracking tools, canvas fingerprints are used to build profiles of users based on the websites...