I will explain security issues related to the construction of web applications using HTML5 based on information presented in the official report on this topic published by JPCERT/CC in October 2013. The official report published by JPCERT/CC in October 2013 on security issues related to the use of HTML5 in web application development aimed to present fundamental information on the latest expertise in this area as systematically as possible for web security researchers as well as application developers. This presentation will include demonstrations and explanations related to the current situation of security issues with web application developed in HTML5 as well as important points that need to be taken into consideration when developing the following functions and components. ・XMLHttpRequest ・HTML new features and attributes ・Browser security functions etc.