Meet the Online Tracking Device That is Virtually Impossible to Block, by Julia Angwin, ProPublica

(This story was co-published with Mashable.)

http://tinyurl.com/mbqqrw

Update: After this article was published, YouPorn contacted us to say it had removed AddThis technology from its website, saying that the website was ‘completely unaware that AddThis contained a tracking software that had the potential to jeopardize the privacy of our users.’  A spokeswoman for the German digital marketer Ligatus also said that is no longer running its test of canvas fingerprinting, and that it has no plans to use it in the future.

A new, extremely persistent type of online tracking is shadowing visitors to thousands of top websites, from WhiteHouse.gov to YouPorn.com.

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.

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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. . . .