Browsers such as Internet Explorer and Firefox give away enough info for web sites to accurately identify you up to 94 per cent of the time, according to recent research.
Peter Eckersley, senior staff technologist at the Electronic Frontier Foundation, says research carried out over the last few months confirms what security experts have known for years.
Browsers give away configuration information such as the type of browser used, operating system, language, time zone details, and what plug-ins and fonts users have installed. This helps to create a unique "fingerprint" for each visitor's computer – which can then be used to track them when they visit other web sites.
"Even if you turn off cookies and you use a proxy to hide your IP address, you could still be tracked," Eckersley told tech mag Computer World.
And, contrary to popular belief, the ‘private browsing’ mode offered by some browsers won’t stop systems from being probed.
Companies such as 41st Parameter, ThreatMetrix, and Iovation already offer cookie-less web tracking to help banks, e-commerce sites and social networks identify fraudsters.
Last August, ThreatMetrix helped movie site iReel nab a single user who was trying out hundreds of different stolen credit cards – even though the individual was using proxy IP addresses.
The EFF has set up a test site to let visitors see what uniquely identifiable information they’re giving away.
Anonymising software such as Tor can help to block the efforts of snoopers by bouncing your data round a network of other machines.