Completely Automated Public Turning Test to tell Computers and Humans Apart (CAPTCHA) is essentially used to prevent some kind of malicious programs from accessing the web resource automatically. In modern era web services is an essential business tool in e-commerce. The emergence of intelligent, complicated attacks similar to DoS (Denial of Service) attack formulates web services still vulnerable. To moderate this attack, CAPTCHA is used as one of the most standard security technology. It is from uncomplicated ones, like typing the imprecise text also with differentiate an object in an image.