Researchers at Princeton and Belgium's KU Leuven have found a new, extremely persistent type of web-tracking technique that's being used on around 5% of the top 1000 sites on the Internet, including, YouPorn, and more. Dubbed Canvas Fingerprinting, the technique is nearly impossible to block, which means you cannot prevent it by blocking cookies, invoking Do Not Track, and using anti-tracking tools such as AdBlock Plus.

The technique works by instructing the visitor's web browser to draw a hidden image. Since each computer renders the image in a different way (see Device fingerprint), that drawing is used to assign the device a uniquely identifying number that allows trackers keep an eye on your browsing activity.

Researchers claim that the most widely used fingerprinting software belongs to a company called AddThis, which started testing the web-tracking technique earlier this year, rolling out the feature to a small portion of the 13 million websites on which its technology appears.

"We're looking for a cookie alternative", said Rich Harris, chief executive of AddThis, adding that the company uses the collected data for internal research and development.

A company spokeswoman said that they do not use any of the data collected from government websites, but gave no such assurances about other websites. The company, however, claims that the collected data will not be used for ad targeting if users install the AddThis opt-out cookie.

It is still unclear just how capable or effective the technology is, but AddThis says that the company may wind things down soon as the results are "not uniquely identifying enough".