In September this year, fraudulent withdrawals related to electronic payment services were discovered one after another, and it became a big topic. Behind this is that the COVID-19 (COVID-19) has increased the number of opportunities for online shopping and cashless payments this year, and various cyber risks aimed at this change have increased. Due to the increase in phishing scams and information leaks not only in Japan but also around the world, password list-type attacks are expected to become more active in the future, and the damage of account takeover and unauthorized use is expected to increase. As the importance of utilizing the web in business growth continues to increase year by year, measures for web security can be said to be an urgent issue. In this book, we will update the latest information on "online fraud", "reliance on third-party resources and web skimming", and "container technology", which are changing rapidly, from the contents released in fiscal 2019, explain the observed attack methods targeting websites, and introduce measures required in the future.