Gotta CAPTCHA ’Em All: A Survey of 20 Years of the Human-or-computer Dilemma

A recent study has found that malicious bots generated nearly a quarter of overall website traffic in 2019 [100]. These perform activities such as price and content scraping, account creation takeover, credit card fraud, denial service, etc. Thus, they represent serious threat to all businesses general, but are especially troublesome for e-commerce, travel financial services. One the most common defense mechanisms against abusing online services is introduction Completely Automated Public Turing test tell Computers Humans Apart (CAPTCHA), so it extremely important understand which CAPTCHA schemes have been designed their actual effectiveness ever-evolving bots. To this end, work provides an overview current state-of-the-art field defines new classification includes emerging schemes. In addition, each identified category, successful attack methods summarized by also describing how evolved resist bot attacks, discussing limitations different from security, usability compatibility point view. Finally, assessment open issues, challenges, opportunities further provided, paving road toward design next-generation secure user-friendly