Vulnerability of the conventional accessible CAPTCHA used by the White House and an alternative approach for visually impaired people

Many people with visual impairments complain about the poor accessibility of conventional CAPTCHA systems because the audio-style test is too difficult for humans. Even a U.S. governmental site, the “We the People” public website, was criticized for the same reason, and thus it implemented a more accessible quiz-based CAPTCHA system. However, this system is vulnerable to simple heuristics. In this study, we demonstrate the insecurity of this type of CAPTCHA system. We demonstrate that our solver program can beat the CAPTCHA with a success rate of over 99%. In addition, we propose a new verbal-style system to replace the quiz-based CAPTCHA. Our system synthesizes several sentences, which have different degrees of naturalness in terms of their contextual meaning, from a set of source documents using a flexible-order Markov chain. Only human users can perceive the difference in the semantics and select the most (or the least) meaningful option correctly. This test is implemented in a verbal style, which means that it is universally suitable for any type of perceptual channel. We implemented our proposed scheme and analyzed its security based on experiments.