[...] A new, extremely persistent type of online tracking is shadowing visitors to thousands of top websites, from WhiteHouse.gov to YouPorn.com. [...]
[It] 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.
[...] They can’t be prevented by using standard Web browser privacy settings or using anti-tracking tools such as AdBlock Plus.