The browser has evolved from a simple program that displays static web pages into a continuously-changing platform that is shaping the Internet as we know it today. The fierce competition among browser vendors has led to the introduction of a plethora of features in the past few years. At the same time, it remains the de facto way to access the Internet for billions of users. Because of such rapid evolution and wide popularity, the browser has attracted attackers, who pose new threats to unsuspecting Internet surfers.

In this dissertation, I present my work on securing the browser against current and emerging threats. First, I discuss my work on honeyclients, which are tools that identify malicious pages that compromise the browser, and how one can evade such systems. Then, I describe a new system that I built, called Revolver, that automatically tracks the evolution of JavaScript and is capable of identifying evasive web-based malware by finding similarities in JavaScript samples with different classifications. Finally, I present Hulk, a system that automatically analyzes and classifies browser extensions.