The vulnerabilities and status of CAPTCHAs

This project's purpose is to give some ideas of Completely Automated Turing Test to Tell Computers and Humans Apart(CAPTCHA), an automatically Turing test which tell human and machine apart, vulnerabilities and its state of the art. As you know, CAPTCHAs are popular in network security area and has been lasting for nearly ten years. However, more and more schemes are suffering successful attacks. Evan Google and Microsoft products cannot slip through those annoying things. The situation of CAPTCHAs seems to be worse and worse. People worry about that and some of them have beginning to believe CAPTCHAs are not as useful as they were stated. Of course, the alternatives of CAPTCHAs are tirelessly emerged one by one. I am also not sure where will CAPTCHAs go tomorrow, but at least as the beginning stated CAPTCHAs are based on hardness AI problems. So, there are still a lot of those kinds of problems can be used and this "arm race" will continue for a long time. Furthermore, we should remember that, the basic idea of CAPTCHAs is tell human and machine apart. So, either the new CAPTCHAs or those alternatives should be created from this basic idea.