CAPTCHAs have been widely used as a defense against automatic attacks. In this interesting research, it has been shown that CAPTCHAs - like any other security mechanisms - are not the final solution!
We report a novel attack on two CAPTCHAs that have been widely deployed on the Internet, one being Google's home design and the other acquired by Google (i.e. reCAPTCHA). With a minor change, our attack program also works well on the latest ReCAPTCHA version, which uses a new defence mechanism that was unknown to us when we designed our attack. This suggests that our attack works in a fundamental level. Our attack appears to be applicable to a whole family of text CAPTCHAs that build on top of the popular segmentation-resistant mechanism of "crowding character together" for security. Next, we propose a novel framework that guides the application of our well-tested security engineering methodology for evaluating CAPTCHA robustness, and we propose a new general principle for CAPTCHA design.
According to the paper, a CAPTCHA must be "segmentation resistant", which refers to the complexity of locating individuals characters (rather than recognizing what a character is) for an attack software.