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Tsuyoshi Arai, Yasuo Okabe, Yoshinori Matsumoto, Koji Kawamura.  2020.  Detection of Bots in CAPTCHA as a Cloud Service Utilizing Machine Learning.

In recent years, the damage caused by unauthorized access using bots has increased. Compared with attacks on conventional login screens, the success rate is higher and detection of them is more difficult. CAPTCHA is commonly utilized as a technology for avoiding attacks by bots. But user's experience declines as the difficulty of CAPTCHA becomes higher corresponding to the advancement of the bot. As a solution, adaptive difficulty setting of CAPTCHA combining with bot detection technologies is considered. In this research, we focus on Capy puzzle CAPTCHA, which is widely used in commercial service. We use a supervised machine learning approach to detect bots. As a training data, we use access logs to several Web services, and add flags to attacks by bots detected in the past. We have extracted vectors fields like HTTP-User-Agent and some information from IP address (e.g. geographical information) from the access logs, and the dataset is investigated using supervised learning. By using XGBoost and LightGBM, we have achieved high ROC-AUC score more than 0.90, and further have detected suspicious accesses from some ISPs that has no bot discrimination flag.