A Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) is a challenge-response integrated within the web forms to determine the user is a human or a spam bot. The CAPTCHA is used to protect a web form against illegal submissions made by a spam bots.A Spam bot is an automated scripts that generate spam content and post it everywhere they can.Usually an image is used to show some distorted text and an input field, asking user to type the challenging text what they see in the distorted text.

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