Precursory Analysis of Attack-Log Time Series by Machine Learning for Detecting Bots in CAPTCHA

CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is commonly utilized as a technology for avoiding attacks to Web sites by bots. State-of-the-art CAPTCHAs vary in difficulty based on the client's behavior, allowing for efficient bot detection without sacrificing simplicity. In this research, we focus on detecting bots by supervised machine learning from access-log time series in the past. We have analysed access logs to several Web services which are using a commercial cloud-based CAPTCHA service, Capy Puzzle CAPTCHA. Experiments show that bot detection in attacks over a month can be performed with high accuracy by precursory analysis of the access log in only the first day as training data. In addition, we have manually analyzed the data that are found to be False Positive in the discrimination results, and it is found that the proposed model actually detects access by bots, which had been overlooked in the first-stage manual discrimination of flags in preparation of training data.