Such "device fingerprinting" circumvents legal restrictions imposed on the use of tracking cookies and ignores the Do Not Track setting in browsers, computer scientists at KU Leuven reported Thursday.
The finding that hundreds of websites were secretly tracking visitors suggests the practice is more widespread than previously thought, the researchers said.
Device fingerprinting, also known as browser fingerprinting, is the practice of collecting technical properties of PCs, smartphones and tablets to identify users.
A 2010 study by the Electronic Frontier Foundation showed that for the vast majority of browsers the combination of these properties is unique, and serves as a fingerprint that can be used to track users.
Device fingerprinting can be used for various legitimate security-related tasks including fraud detection protection against account hijacking but it is also being used for marketing purposes via fingerprinting scripts hidden in advertising banners and web widgets, the researchers said.
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