“Most people are familiar with image-based CAPTCHAs that require users to input a string of distorted characters in order to prove that they are human. However, there are also audio and video variants of such tests.
NuCaptcha is a video-based CAPTCHA implementation that uses animation techniques in order to make it harder for spam bots to decipher the characters. Its creators claim that NuCaptcha has the highest usability and security levels of any CAPTCHA on the market.
However, according to Stanford University researcher Elie Bursztein, that’s not exactly true. Bursztein has worked with other researchers to evaluate the security of NuCaptcha since October 2010 and has devised a method that defeats it with a success rate of over 90 percent.” (Lucian Constantin, Network World)