  For the last ten years, CAPTCHAs have been widely used by websites to prevent
their data being automatically updated by machines. By supposedly allowing only
humans to do so, CAPTCHAs take advantage of the reverse Turing test (TT),
knowing that humans are more intelligent than machines. Generally, CAPTCHAs
have defeated machines, but things are changing rapidly as technology improves.
Hence, advanced research into optical character recognition (OCR) is overtaking
attempts to strengthen CAPTCHAs against machine-based attacks. This paper
investigates the immunity of CAPTCHA, which was built on the failure of the TT.
We show that some CAPTCHAs are easily broken using a simple OCR machine built
for the purpose of this study. By reviewing other techniques, we show that even
more difficult CAPTCHAs can be broken using advanced OCR machines. Current
advances in OCR should enable machines to pass the TT in the image recognition
domain, which is exactly where machines are seeking to overcome CAPTCHAs. We
enhance traditional CAPTCHAs by employing not only characters, but also natural
language and multiple objects within the same CAPTCHA. The proposed CAPTCHAs
might be able to hold out against machines, at least until the advent of a
machine that passes the TT completely.
