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Thus, they represent a serious threat to all businesses in general, but are especially troublesome for e-commerce, travel, and financial services. One of the most common defense mechanisms against bots abusing online services is the introduction of Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA), so it is extremely important to understand which CAPTCHA schemes have been designed and their actual effectiveness against the ever-evolving bots. To this end, this work provides an overview of the current state-of-the-art in the field of CAPTCHA schemes and defines a new classification that includes all the emerging schemes. In addition, for each identified CAPTCHA category, the most successful attack methods are summarized by also describing how CAPTCHA schemes evolved to resist bot attacks, and discussing the limitations of different CAPTCHA schemes from the security, usability, and compatibility point of view. Finally, an assessment of the open issues, challenges, and opportunities for further study is provided, paving the road toward the design of the next-generation secure and user-friendly CAPTCHA schemes.<\/jats:p>","DOI":"10.1145\/3477142","type":"journal-article","created":{"date-parts":[[2021,10,8]],"date-time":"2021-10-08T22:02:57Z","timestamp":1633730577000},"page":"1-33","update-policy":"https:\/\/doi.org\/10.1145\/crossmark-policy","source":"Crossref","is-referenced-by-count":47,"title":["Gotta CAPTCHA \u2019Em All: A Survey of 20 Years of the Human-or-computer Dilemma"],"prefix":"10.1145","volume":"54","author":[{"given":"Meriem","family":"Guerar","sequence":"first","affiliation":[{"name":"DIBRIS, University of Genoa, Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luca","family":"Verderame","sequence":"additional","affiliation":[{"name":"DIBRIS, University of Genoa, Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mauro","family":"Migliardi","sequence":"additional","affiliation":[{"name":"Department of Electronic Engineering, University of Padua, Padua, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Francesco","family":"Palmieri","sequence":"additional","affiliation":[{"name":"Department of Computer Science, University of Salerno, Salerno, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Alessio","family":"Merlo","sequence":"additional","affiliation":[{"name":"DIBRIS, University of Genoa, Genoa, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"320","published-online":{"date-parts":[[2021,10,8]]},"reference":[{"key":"e_1_2_1_1_1","unstructured":"josscrowcroft.com. 2011. MotionCAPTCHA v0.2 Stop Spam Draw Shapes. Retrieved from http:\/\/www.josscrowcroft.com\/demos\/motioncaptcha\/.  josscrowcroft.com. 2011. MotionCAPTCHA v0.2 Stop Spam Draw Shapes. Retrieved from http:\/\/www.josscrowcroft.com\/demos\/motioncaptcha\/."},{"key":"e_1_2_1_2_1","volume-title":"International Conference on Computational Science and Computational Intelligence (CSCI\u201919)","author":"Ababtain E.","year":"2019","unstructured":"E. Ababtain and D. Engels . 2019. Gestures based CAPTCHAs the use of sensor readings to solve CAPTCHA challenge on Smartphones . In International Conference on Computational Science and Computational Intelligence (CSCI\u201919) . 113\u2013119. DOI:https:\/\/doi.org\/10.1109\/CSCI49370. 2019 .00026 10.1109\/CSCI49370.2019.00026 E. Ababtain and D. Engels. 2019. Gestures based CAPTCHAs the use of sensor readings to solve CAPTCHA challenge on Smartphones. 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