CAPTCHAs and the Robot in the Middle attack

A CAPTCHA is a visual test of humanity used to prevent machines from performing an operation that is intended to be performed only by people. Many internet services use this to prevent mass automatic access to their services. For example, Google requires anyone registering a Gmail account to pass a CAPTCHA test.

The most commonly used CAPTCHA is a request to identify letters that are presented on the screen in a form which is difficult for OCR software to identify - see example on the right.

One way to circumvent CAPTCHAs is to use a Robot-in-the-Middle attack.