W3C logoWeb Accessibility initiative

WAI: Strategies, guidelines, resources to make the Web accessible to people with disabilities

Introduction to "Inaccessibility of CAPTCHA"

Example CAPTCHA Form

the letters 'captcha' difficult to read because skewed on complex background

NOTE: "Inaccessibility of CAPTCHA" was written in 2005 and may not reflect current best practices.

The W3C Note Inaccessibility of CAPTCHA W3C Note examines potential solutions to test that users are human, not software robots, in a way that is accessible to people with disabilities.

Robot or Human? Tests Discriminate Against Humans

CAPTCHA stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart. CAPTCHA and other security tests are designed to block software robots from interacting with a Web site. Most of these tests also block humans who are blind, deaf, hard of hearing, have low vision, or a cognitive/intellectual disability such as dyslexia.

For example, a common test requires users to read a distorted set of characters from a graphic image and enter the characters into a form. This test is not accessible to people who are blind and people with some types of low vision or cognitive disabilities such as dyslexia.

There are other types of tests to block software robots that may be as effective and more accessible to people with disabilities. The W3C Note Inaccessibility of CAPTCHA W3C Note discusses the pros, cons, and accessibility issues of several other tests.

W3C Note Format

Inaccessibility of CAPTCHA W3C Note follows the W3C Note format which includes several sections at the beginning: links to different versions, editor, copyright, abstract, and status. It does not include the WAI Web site navigation.