Abstract
the internet these days plays an important role in our daily lives, as the number of web services are increasing such as search engines and email, they are threatened by different types of attacks from computer programs such as robots. In order to solve this problem, CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) was implemented to be used as a base to distinguish between human users and computer bots. Although this technique limits the automatic registration to web services to provide security but, still some CAPTCHAs have several weaknesses which allow the computer bots to infiltrate the mechanism of the CAPTCHA. The aim of this paper is to search for the problems in the existing types of CAPTCHA systems and to provide solutions or alternative approaches to solve these problem.