A Survey of Current Research on CAPTCHA

The internet has been playing an increasingly important role in our daily life, with the availability of many web services such as email and search engines. However, these are often threatened by attacks from computer programs such as bots. To address this problem, CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart) was developed to distinguish between computer programs and human users. Although this mechanism offers good security and limits automatic registration to web services, some CAPTCHAs have several weaknesses which allow hackers to infiltrate the mechanism of the CAPTCHA. This paper examines recent research on various CAPTCHA methods and their categories. Moreover it discusses the weakness and strength of these types.