Web browser security is a hot and important research area. If a web browser is vulnerable, users can be affected by malware without their knowledge. In this presentation, we will introduce methods to find vulnerabilities in Javascript Engines for web browsers via fuzzing. We will also reveal our own crash classification method and parallel fuzzing framework. We will share some tips for patternizing the results of 1-day case analysis for creating random javascript syntax and our custom pattern L.E.G.O.”]