1. A CAPTCHA method executed by a CAPTCHA system, comprising: receiving a CAPTCHA request comprising category information of an application service from an application server; responding to the application server with a token identifying the CAPTCHA request and a CAPTCHA image comprising a distorted advertisement word associated to the category information and a series of randomly generated and distorted characters, both the advertisement word and the characters being a CAPTCHA text intended to be typed by a user via a user equipment connected to the application server; receiving from the application server the token and a CAPTCHA answer submitted by the user; and verifying the token and the answer and returning to the application server a result of the verification.
6. The method according to claim 1 , wherein the CAPTCHA answer and the token are stored in a cache in the CAPTCHA system, and fetched only once and removed from the cache instantly after being fetched.