Browsing in "private mode" isn't as private as users think, a researcher said today.

"There are some traces left behind [by all browsers] that could reveal some of the sites that you've been to," said Collin Jackson, an assistant research professor at the Silicon Valley campus of Carnegie Mellon University. Jackson, along with three colleagues from Stanford University, will present their findings later today at the Usenix Security Symposium in Washington, D.C.

Internet Explorer (IE), Firefox, Chrome and Safari offer private browsing intended to cloak a user from Web sites and erase all browsing evidence from the PC or Mac.

Apple's Safari was the first browser to feature private browsing, followed in 2008 by Google's Chrome, and in the next year, by Microsoft's IE and Mozilla's Firefox. Opera added a similar feature this summer, but was not included in the research.

The tools are designed primarily to prevent Web destinations from being recorded to the browser's history so that someone else with access to the computer -- a spouse, say, or child -- can't see where the browser's been. Chrome, calls its feature "Incognito," while IE dubs it "InPrivate."

"All the browsers are trying to protect you from this local attacker," said Jackson, using the term he and his fellow researchers applied to others who could put hands on the computer's keyboard or otherwise access the machine.

IE, Firefox and Safari, for instance, leave traces of SSL (secure socket layer) encryption keys even when run in private mode, while IE and Safari on Windows preserve self-signed SSL certificates in a "vault" file that could be read by others to track the browser's path across the Web.

Firefox also retains evidence of some certificates, particularly non-standard certificates used by some government agencies, in a file that can be mined by others, the four researchers said in their paper.

Private mode has also been billed by browser makers as a way for users to hide themselves from the prying eyes of sites that try to track habits and histories. In fact, said Jackson, most users see that as the biggest attraction to private mode.

"Some browsers do a better job of protecting you from other types of scenarios, such as Web site tracking," Jackson said. "Safari is very much more willing to reveal you to Web sites than the others."

Safari uses a completely different model than its rivals, one that makes available to sites virtually all public information during private sessions. Jackson declined to criticize Apple for how it handles private mode, insisting that it was "simply different" from the others.