Even without cookies, popular browsers such as Internet Explorer and Firefox give websites enough information to get a unique picture of their visitors about 94 percent of the time, according to research compiled over the past few months by the Electronic Frontier Foundation (EFF).
The research puts a quantitative assessment on something that security gurus have known about for years, said Peter Eckersley, the EFF senior staff technologist who did the research. He found that configuration information - data on the type of browser, operating system, plugins, and even fonts installed can be compiled by websites to create a unique portrait of most visitors.
This means that most internet users are a lot less anonymous than they believe, Eckersley said. "Even if you turn off cookies and you use a proxy to hide your IP address, you could still be tracked," he said.
The data doesn't actually identify the web user, but it creates a unique browser 'fingerprint', that can be used to identify the user when he visits other websites.
And using the private mode offered by some browser-makers does nothing to stop this analysis. "They provide you with some protection against other people who may be in your house or who have access to your computer, but they haven't got to the point where they've provided protection against the companies that are profiling Web users," Eckersley said.
In fact, there are already a handful of companies have already started offering this kind of cookie-less web tracking to help e-commerce sites identify fraudsters. Companies such as 41st Parameter, ThreatMetrix, and Iovation are widely used in the banking, e-commerce and social websites.
And the products work. Last August, when Serbian criminals started testing stolen credit cards by posting hundreds of $1.99 transactions to the iReel.com online movie site each day, iReel turned to ThreatMetrix to get a fix on the fraudsters.
Using similar techniques to those described by the EFF, ThreatMetrix generated digital fingerprints of site visitors, which helped iReel know when a single user was trying to use hundreds of different credit cards, even when the fraudster was using proxy IP addresses, said Adam Altman, iReel's chief operations officer. "We were able to cut out a lot of the unnecessary transactions," he said.
These products may see more widespread use as browser-makers give users more control over managing the web-tracking cookie files on their browsers, said Avivah Litan, an analyst with Gartner. Many e-commerce sites already use so-called Flash cookies to track visitors, but Adobe is starting to give users more control over these files, so browser fingerprinting maybe the next widely used visitor tracking technology on the web, she said.
For people who think they're anonymously surfing the web, this is bad news, Eckersley said. "If someone can see what pages we're going to, they know what we're reading and what we're thinking," he said.
The EFF has set up a website that tests visitors for uniquely identifiable information. Most people are surprised to discover just how trackable they are online, Eckersley said.
There are some effective countermeasures, however. A uniquely identifiable Windows XP computer we tested running Firefox could not be identified with the NoScript safe browsing extension turned on. Adding the Tor internet anonymisation software also works, Eckersley said.
Mobile browsers on the iPhone on Android platforms are often not identifiable. That's because they typically don't have the variety of browser plugins and font add-ons that are common on desktop PCs.
So could browser makers make their products more private? Eckersley believes so. "There are some situations where having some of this information is useful, but you don't need all of it by any means," he said.