CAPTCHA is an acronym for “completely automated public Turing test to tell computers and humans apart."
A CAPTCHA is a type of challenge-response test used in computing to determine whether the user is human. For example:
Why is this needed on websites? CAPTCHA is used to prevent bots from automatically submitting forms with SPAM or other unwanted content. Google and other companies use it to prevent bots from creating multiple Gmail accounts.
A CAPTCHA is used to prevent bots from hacking into your account. Which leads to the next question, what is a bot? A bot, aka web robot, is a software application that runs automated tasks over the Internet.
These bots use different types of attacks. One frequently used attack is referred to as the "dictionary" attack. This type of attack uses a program designed to crack your password. It does this by trying thousands or millions of likely password possibilities. For example, words in a dictionary or previously used passwords. Another type of attack used is called a brute-force attack. It consists of an attacker submitting many passwords or passphrases with the hope of eventually making a correct guess.
One of the most important reasons for CAPTCHA is to defend against ad spammers who promote their scams in comments on web pages. By requiring all users to negotiate the CAPTCHA authentication, administrators can filter out spammers who attempt to automate their activities.
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