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 they visit - profiles that shape which ads, news articles, or other types of content are displayed to them. But fingerprints are unusually hard to block: They can't be prevented by using standard Web browser privacy settings or using anti-tracking tools such as AdBlock Plus.The researchers found canvas fingerprinting computer code, primarily written by a company called AddThis, on 5 percent of the top 100,000 websites. Most of the code was on websites that use AddThis' social media sharing tools. Other fingerprinters include the German digital marketer Ligatus and the Canadian dating site Plentyoffish. (A list of all the websites on which researchers found the code is here).
Rich Harris, chief executive of AddThis, said that the company began testing canvas fingerprinting earlier this year as a possible way to replace "cookies," the traditional way that users are tracked, via text files installed on their computers.