Study Finds That Caching by Browsers Creates a Threat to Surfers' Privacy Browsers speed up Web use by storing, or caching, recently viewed Web pages, or at least elements of them,
on their user's hard drive. The next time the user summons that page, the browser speeds up its retrieval by
pulling it from the hard drive rather than going out and extracting it from the Internet. (Not all pages can be
cached. Sports, news, weather and other pages that are frequently updated are designed in a way to block the
process, at least for content. But portions of even these pages, like logos and page design elements, are
generally cached to help speed up downloads.)

As a browser cache gradually accumulates pages, it effectively becomes a detailed electronic record of its user's
Web browsing. For that reason, Web browsers have several security elements to prevent outsiders from
reading their contents.

The cache attack method discovered by Dr. Felten and Mr. Schneider does not break that security barrier.
What they have found, however, is a way for outsiders to probe caches to see if they hold files from specific
Web pages. If attackers find the files they're seeking, it's a clear sign that the browser user has recently visited
the site that produced those files.
http://www.nytimes.com/2000/12/14/technology/14PRIV.html?pagewanted=all