Inaccessibility of CAPTCHA

Various approaches have been employed over many years to distinguish human users of web sites from robots. The traditional CAPTCHA approach asking users to identify obscured text in an image remains common, but other approaches have emerged. All interactive approaches require users to perform a task believed to be relatively easy for humans but difficult for robots. Unfortunately the very nature of the interactive task inherently excludes many people with disabilities, resulting in a denial of service to these users. Research findings also indicate that many popular CAPTCHA techniques are no longer particularly effective or secure, further complicating the challenge of providing services secured from robotic intrusion yet accessible to people with disabilities. This document examines a number of approaches that allow systems to test for human users and the extent to which these approaches adequately accommodate people with disabilities, including recent non-interactive and tokenized approaches. We have grouped these approaches by two category classifications: Stand-Alone Approaches that can be deployed on a web host without engaging the services of unrelated third parties and Multi-Party Approaches that engage the services of an unrelated third party.