CAPTCHA: accessibility for developers

  • Author: Access iQ ®
  • Date: 1 Feb 2013
  • Access: Premium

Quick facts

A well-known method of preventing unwanted form submissions is to use a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) in order to avoid input that has been generated by a computer or "bots".


  • Before implementation, consider whether a CAPTCHA is necessary and if an alternative is available.
  • Research options before choosing a solution.
  • Look at different types of CATPCHAs: reCAPTCHA, Text-only CAPTCHAs, Honeypot CAPTCHAs.

On many websites, it is common to ask visitors to supply information or content via a web form. These forms are vulnerable because they present an opportunity for bots to automatically fill in and submit forms. These spam submissions can vary from relatively benign skewing of opinion polls or placing advertising into comments to more damaging vandalism, spreading malicious code or stealing private information.

A well-known method of preventing unwanted form submissions is to a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) in order to avoid input that has been generated by a computer or "bots". CAPTCHAs do this by giving a user some sort of challenge that would be difficult for an automated bot to complete successfully.

The most common form of CAPTCHA is illustrated below, where distorted text is presented as an image. The user is expected to type the characters they see into a form field. The assumption is that most bots are not capable of recognising or interpreting the distorted text and will fail the CAPTCHA.

People with an impairment or disability may not be able to pass this sort of CAPTCHA.

A CAPTCHA using a single word verification with distorted text presented as an image.

People with an impairment or disability may not be able to pass this sort of CAPTCHA.

This includes people who:

  • are blind
  • are vision impaired
  • are dyslexic
  • have difficulty reading

The difficulty in making a CAPTCHA image accessible is that providing a text alternative of the image, as required for screen reader users to understand the content, also supplies the answer to the bot.

Success Criterion 1.1.1 Non-text content explicitly excludes the requirement to supply a text alternative to a CAPTCHA image for this very reason. But that doesn't let the developer off the hook, because it required that:

…text alternatives that identify and describe the purpose of the non-text content are provided, and alternative forms of CAPTCHA using output modes for different types of sensory perception are provided to accommodate different disabilities.

This can be satisfied by providing a text alternative that identifies the purpose of the CAPTCHA, and ensuring the page provides another CAPTCHA that serves the same purpose but is perceivable by a different sense. For example, by presenting a test that depends on hearing, not sight. It should be noted that the audio test does not have to replicate the visual test verbatim, it merely needs to have the same amount of distortion to the test.

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