Browser Fingerprinting: 9 Facts

Tracking technology that can identify individual identities and devices is improving faster than consumers might realize, warn privacy researchers.

Mathew J. Schwartz, Contributor

October 24, 2013

9 Min Read

Not all firms that track consumers' browsing behavior by using advanced browser fingerprinting techniques fail to honor "Do Not Track" (DNT) flags or opt-out preferences.

So said James Brentano, VP of solutions at BlueCava, responding to a recently released study -- "FPDetective: Dusting the Web for Fingerprinters" -- from privacy researchers in Belgium and the United States. The study reported that at least 404 of the world's one million most popular websites were using advanced techniques to fingerprint users and devices and to tie an individual consumer's identity to multiple devices. The researchers warned that latest-generation techniques, based on JavaScript and Flash fingerprinting of devices, aren't technically cookies, and thus might allow advertisers to bypass European cookie laws.

But Brentano said that BlueCava -- whose fingerprinting tracking technology the researchers most often encountered during their survey of the Web -- doesn't fingerprint in a surreptitious manner. "We do respect 'Do Not Track' from all the browsers. We do have opt out," he said, speaking by phone. "There's no value for a company like us in tracking people who don't want to be tracked, because people who don't want to be tracked don't respond to tracking."

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Still, few consumers likely know about browser fingerprinting, and as awareness grows, the topic promises to become contentious. Here are nine related facts to understand as this debate unfolds:

1. Multiple Tracking Firms Employ Fingerprinting Techniques.

The researchers behind the FPDetective study reported finding fingerprinting technology from numerous firms, including Bitcoin digital wallet provider CoinBase, geolocation and "online fraud prevention" firm MaxMind, consumer tracking provider Mindshare Technology, as well as services with such names as Analyticsengine, Anonymizer, fingerprint.js, Inside graph and Perferencement. But they wrote that BlueCava's font-probing JavaScript code was the most prevalent, and "the only one of the discovered font-probing scripts that queries different sets of fonts based on the device's operating system: 231 fonts for Microsoft Windows, 167 for Mac OS and 62 for other operating systems."

Brentano said this fingerprinting is designed to identify a given device, but not to surreptitiously track it. "Commercially -- and I don't know what the bad guys are doing -- but there's no intent to bypass a user's preferences," he said. "This isn't about privacy, this is about economics. The goal is to give users choice, which sounds like marketing crap, but it's economically true. There's no value to trying to track a user who objects. Brands are very explicit about this: our customers put the burden on us, make sure users know this is happening, and can opt out."

2. Most Consumers Don't Understand Fingerprinting.

Brentano also said that the browser-fingerprinting techniques -- for example, making a record of the fonts used by a given computer -- are well-known in the advertising and tracking industries. "Everyone in this space pretty much has access to the same information -- you can see the fonts, the user agent," he said. But he noted that browsers will also change over time, meaning that the profile of a given device must evolve. "The secret sauce, if you will, is to be able to take these two profiles and recognize if they're the same [device], because you have to do it in Internet time."

Privacy advocate Jim Brock, however, said via phone that he didn't think these types of fingerprinting techniques have been widely adopted. "I'm glad [BlueCava has] an opt-out program; that's good. I'm glad they have a reset button; that's good. But I do not think it's mainstream ... what they're doing," said Brock, who founded PrivacyChoice in 2009, which was acquired by AVG Technologies in May 2013. Brock currently serves as VP of privacy products at AVG.

Gunes Acar, lead author of the FPDetective paper and Ph.D. student -- researching Web and mobile application privacy -- at the University of Leuven in Belgium, posited that most consumers would be surprised to learn about these fingerprint techniques, which were first discovered by a font geek. "I don't think it's well known, even in academia," Acar said via email. "Most of the people who hear about that -- measuring the sizes of invisible strings with different fonts -- freak out."

3. Billions In Ad Revenue Drive Consumer Tracking.

The economic incentives to track users today are higher than ever. Internet sales figures from the first half of 2013 totaled $20.1 billion -- an all-time high -- which was an increase of 18% from the same period last year.

Still, what's wrong with fingerprinting techniques? "My problem with them is they're immutable, invisible and unexpected by consumers," Brock explained. "These types of methods are on the frontier of aggressive data collection because ... they associate your data and activity across multiple devices, and associate your household's devices in a way that consumers wouldn't expect."

4. Not All Fingerprinting Vendors Are The Same.

As demand for new tracking techniques grows, however, not all JavaScript and Flash-based fingerprinting technology -- or vendors -- are the same. "Let me acknowledge that among the fingerprinting companies we aware of, BlueCava might be the most transparent about their practices," Acar said. "I guess this is partly because they want to operate in Europe and have to comply with the EU directives." These include the eCookie Directive, which was designed to ensure that users were tracked only with their consent.

Legally speaking, fingerprinting technology falls into a gray area. "Since you don't have to store cookies with fingerprinting, user consent is possibly not required," Acar said, though he noted that this has yet to be tested in European court. In addition, he noted that BlueCava's opt-out page doesn't apply to third parties who use its technology, which may include for fraud prevention purposes. 5. Users Can Block Fingerprinting – Sometimes.

Bretano said BlueCava's fingerprinting isn't hidden from browser privacy plug-ins designed to track tracking technology. "I can only speak for us, but the most common tool, Ghostery, absolutely sees us, they will see our code run. We explicitly write a cookie whenever we can, so we leave a mark behind," he said.

But Acar noted that not all tracking technology can be detected by tracking monitoring software such as Ghostery or NoScript. "Ghostery has a big database of trackers, if they add the ones we found to their databases Ghostery can block some of them," he explained. "Still, there are ways to circumvent these protections, like serving the same script from different addresses." In addition, he said, "NoScript can block some fingerprinters -- depends on the configuration."

6. Fingerprinting Can Make "Opt-Out" Preferences Stick.

BlueCava's Brentano said his firm also uses its fingerprinting techniques to ensure that a consumer's opt-out preferences persist. "We believe that we do a better job of opt out, because with cookies, if you opt out, and then delete the cookies -- which people often do -- then you delete your opt out," he said. "But we also record an opt-out event against our record of that device ... and we'll actually reset the opt-out cookie."

But what about giving consumers the right to opt in to these techniques -- rather than being stuck in the situation of having to opt out of techniques they may not realize are being used? "That's an absolutely legitimate political debate, which we do not have an opinion on," Brentano said. "From our standpoint, either one is fine. We just play by the rules that the industry and regulatory regime sets."

7. Do Not Track: Not Mentioned In BlueCava's Privacy Policy.

But AVG's Brock questioned why BlueCava's privacy statement makes no mention of any Do Not Track compliance. "The Federal Trade Commission can only effectively enforce statements that are literally made, and I couldn't find a statement in [BlueCava's] privacy policy that they honor Do Not Track," he said. "So their statement has no legal effect, as far as I know."

8. Are Advertisers Seeking Legal Protection For Fingerprinting?

The Digital Advertising Alliance and the Interactive Advertising Bureau -- both advertising trade groups -- are currently developing standards for all types of tracking, including cookies. They say this will provide consumers with a single, consistent way to opt out of being tracked, although some privacy groups think it may be a push by the industry to legitimize obscure -- and likely controversial -- fingerprinting techniques.

In addition, according to Brock, by combining these techniques, advertisers are gaining new ways to tie together devices with people's identities and personal information. For example, if a user searches for information about a disease on their smartphone, that information could end up getting added to a file -- maintained about that one person -- that gets bought and sold by data brokers, and which also records what they do or see from their PC and tablet.

9. More Aggressive Tracking To Come?

Given the overarching privacy and regulatory questions surrounding tracking, don't expect advanced fingerprinting techniques -- or related debates -- to go away, especially if more people begin to use ad-blocking technology. "We're going to be hearing a lot more about this technology as the advertisers become more desperate," Brock said. "We don't have a Do Not Track standard, and the industry organizations are embracing these new aggressive tracking methods as a way to shore up the business."

Furthermore, tracking firms still have many more tracking techniques available to them, should they decide to use them. "There are ways to fingerprint devices without JavaScript or Flash. Clock skew, network packet fingerprinting and our attack on Tor Browser -- scriptless font fingerprinting -- are examples for passive fingerprinting techniques," said University of Leuven's Acar. "These techniques I'd refer to as really, really stealthy compared to JavaScript or Flash-based fingerprinting. They don't require any client-side code to run and are very hard to detect for researchers too."

About the Author(s)

Mathew J. Schwartz


Mathew Schwartz served as the InformationWeek information security reporter from 2010 until mid-2014.

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