Accessibility

Because CAPTCHAs rely on perception, users unable to perceive a CAPTCHA (for example, due to a disability or because it is difficult to read) will be unable to perform the task protected by a CAPTCHA. As such, sites implementing CAPTCHAs should provide an audio version of the CAPTCHA in addition to the visual method. The official CAPTCHA site recommends providing an audio CAPTCHA for accessibility reasons.

[edit] Attempts at more accessible CAPTCHAs

Even an audio and visual CAPTCHA will require manual intervention for some users, such as those who are both deaf and blind. There have been various attempts at creating CAPTCHAs that are more accessible. Attempts include the use of JavaScript,mathematical questions ("what is 1+1"), or "common sense" questions ("what color is the sky"). These attempts violate one or both of the principles of CAPTCHAs: either they cannot be automatically generated or they can be easily cracked given the state of artificial intelligence. As such, the only security these CAPTCHAs provide is security through obscurity; an attacker is unlikely to have encountered the formulation of the CAPTCHA in question, and unlikely to find it worth the time spending resources to break the CAPTCHA of a small site.

Due to the lack of security provided by text based CAPTCHAs, most sites choose to use an audio and visual CAPTCHA as a way of balancing accessibility and security. Often, email or telephone support is used to manually provide access to users who are unable to solve a CAPTCHA.