By employing people to solve the CAPTCHA problem, for as little as £2 or £3 a day, cyber criminals have access to millions of registered accounts," says Rik Ferguson at Trend Micro.

Take this as an example, hacker puts up a porn site where users have to pass captcha before watching the clip. However, the captcha user passes is used by hacker to solve `captcha` from the other site from where he wants to send spam. Think of it as `captcha phishing` :-p