I. Introduction
Current web browsers provide attackers with easy access to modern computer systems. According to a recent report by Symantec [48], over the last year Internet Explorer had 93 security vulnerabilities, Mozilla browsers had 74 vulnerabilities, Safari had 29 vulnerabilities, and Opera had 9 vulnerabilities. In addition to these browser bugs, there were also 301 reported vulnerabilities in browser plugins over the same period of time including high-profile bugs in the Java virtual machine [10], the Adobe PDF reader [38], the Adobe flash player [8], and Apple's QuickTime [39]. Unfortunately, attackers actively exploit these bugs according to several recent reports [50], [36], [41], [48].