At least 145 of the Internet’s top 10 000 Web sites track users without their knowledge or consent, using hidden scripts to extract a device fingerprint from the users’ browsers, according to a new study by KU Leuven-iMinds.

Device fingerprinting circumvents legal restrictions imposed on the use of cookies and ignores the “Do Not Track” HTTP header.

The findings suggest that secret tracking is more widespread than previously thought.

Device fingerprinting, also known as browser fingerprinting, is the practice of collecting properties of PCs, smartphones and tablets to identify and track users. These properties include the screen size, the versions of installed software and plug-ins, and the list of installed fonts.

A 2010 study by the Electronic Frontier Foundation (EFF) showed that, for the vast majority of browsers, the combination of these properties is unique, and thus functions as a ‘fingerprint’ that can be used to track users without relying on cookies.

Device fingerprinting targets either Flash, the ubiquitous browser plug-in for playing animations, videos and sound files, or JavaScript, a common programming language for Web applications.

This is the first comprehensive effort to measure the prevalence of device fingerprinting on the Internet. The team of KU Leuven-iMinds researchers analysed the Internet’s top 10 000 Web sites and discovered that 145 of them (almost 1,5%) use Flash-based fingerprinting. Some Flash objects included questionable techniques such as revealing a user’s original IP address when visiting a Web site through a third party (a so-called proxy).

The study also found that 404 of the top 1-million sites use JavaScript-based fingerprinting, which allows sites to track non-Flash mobile phones and devices. The fingerprinting scripts were found to be probing a long list of fonts – sometimes up to 500 – by measuring the width and the height of secretly-printed strings on the page.

The researchers identified a total of 16 new providers of device fingerprinting, only one of which had been identified in prior research. In another surprising finding, the researchers found that users are tracked by these device fingerprinting technologies even if they explicitly request not to be tracked by enabling the Do Not Track (DNT) HTTP header.

The researchers also evaluated Tor Browser and Firegloves, two privacy-enhancing tools offering fingerprinting resistance. New vulnerabilities – some of which give access to users’ identity – were identified.

Device fingerprinting can be used for various security-related tasks, including fraud detection, protection against account hijacking and anti-bot and anti-scraping services. But it is also being used for analytics and marketing purposes via fingerprinting scripts hidden in advertising banners and Web widgets.

To detect Websites using device fingerprinting technologies, the researchers have developed a free tool called FPDetective.