We study the security and privacy of private browsing

modes recently added to all major browsers. We first propose

a clean definition of the goals of private browsing

and survey its implementation in different browsers. We

conduct a measurement study to determine how often it is

used and on what categories of sites. Our results suggest

that private browsing is used differently from how it is

marketed. We then describe an automated technique for

testing the security of private browsing modes and report

on a few weaknesses found in the Firefox browser. Finally,

we show that many popular browser extensions and

plugins undermine the security of private browsing. We

propose and experiment with a workable policy that lets

users safely run e