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Private browsing modes in four biggest browsers often fail

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Features in the four major browsers designed to cloak users' browser history often don't work as billed, according to a research paper that warns that users may get a false sense of security when using the built-in privacy settings.

The private-browsing modes are supposed to allow users to visit a website without leaving any trace on their computers, and yet Internet Explorer, Firefox, Chrome, and Safari frequently leave tracks, according to the research, which is scheduled to be presented at next week's Usenix Security Symposium in Washington DC. The makers of those browsers — Microsoft, Mozilla, Google, and Apple respectively — often hail the offerings as a way to enhance privacy when using shared computers.

One failure that affects IE, Firefox, and Safari happens when users save SSL, or secure sockets layer, client certificates while browsing in private mode. The browsers store a record of those actions in a file that allows anyone who has physical access to know exactly what site the user was visiting at the time. Similarly, when IE and Safari encounter a self-signed certificate, it is stored in a certificate vault that is preserved even after the private session ends.

Similarly, Firefox users who make security certificate settings while in private mode will have a partial copy of their browsing history stored in a file called cert8.db, the researchers said.

“We discovered that all these browsers retain the generated key pair even after private browsing ends,” the researchers wrote. “Again, if the user visits a site that generates an SSL client key pair, the resulting keys will leak the site's identity to the local attacker.”

The study (PDF here) showed each browser failing in specific settings.

The privacy mode in Firefox, for instance, is undermined when a user sets site-specific preferences or uses a variety of Mozilla-sanctioned plug-ins. The open-source browser also stores websites visited that dole out custom protocol handlers based on the HTML5 standard.

For its part, IE's InPrivate mode can be undermined when websites make SMB queries, since the Microsoft browser shares large chunks of code with Windows Explorer.

The researchers also devised a way for webmasters to detect when someone visiting their sites is using the privacy mode. It involves placing an iframe with a unique web address and then “using JavaScript to check whether a link to that URL was displayed as purple (visited) or blue (unvisited).”

The researchers said that to the best of their knowledge they are the first to demonstrate a way to detect private browsing mode — but that may not really matter for much longer. The technique appears to use the decade-old browser history attack, which was recently fixed in Safari and will soon be fixed in Firefox. It's only a matter of time before Microsoft and Google follow suit.

Using the technique, they confirmed what we all suspected: the feature is mainly used when surfing to porn sites. Gift and news sites, not so much. ®

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