Google reCAPTCHA.
reCAPTCHA: Easy on Humans, Hard on Bots - Google.
What is reCAPTCHA? reCAPTCHA is a free service that protects your website from spam and abuse. reCAPTCHA uses an advanced risk analysis engine and adaptive challenges to keep automated software from engaging in abusive activities on your site. It does this while letting your valid users pass through with ease.
reCAPTCHA: Easy on Humans, Hard on Bots - Google | How to add reCAPTCHA Hotel Website with a Room Booking Engine.
reCAPTCHA is a CAPTCHA-like system designed to establish that a computer user is human. reCAPTCHA was originally developed by Luis von Ahn, David Abraham, Manuel Blum, Michael Crawford, Ben Maurer, Colin McMillen, and Edison Tan at Carnegie Mellon University's main Pittsburgh campus.
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How to protect Your Hotel PMS Using reCAPTCHA Google: Your Hotel web Booking Engine.
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