A technique that exploits the way Web browsers store recently viewed data could compromise Internet users' privacy by allowing an attacker to check what sites a person has visited recently.

A technique that exploits the way Web browsers store recently viewed data could compromise Internet users' privacy by allowing an attacker to check what sites a person has visited recently. The exploit--called a "timing attack"--allows an unethical Web site to play 20 questions (or more) with a person's browser and check whether the surfer has recently viewed any sites from a predetermined list. An employer could use the technique on internal Web sites to see whether any employees have been visiting the competition's job listings. A Web portal could check whether a person has recently visited any of its sponsors. "The attacks allow any Web site to determine whether or not each visitor has recently visited some other site" or set of sites, Edward Felten, a professor of computer science at Princeton University, and Michael Schneider, a graduate student, wrote in a paper published at a technical conference last month. "The attacker can do this without the knowledge or consent of either the user or the other site," they wrote. The attack takes advantage of the data caches used by browsers to speed access to recently visited Web sites. Caching is a technique that stores copies of frequently accessed data in a nearby location, whether on a person's PC or on a server on the local area network. The ability to store recently viewed items significantly reduces the amount of data that has to move over the Internet.