How do you present CAPTCHA accessibly?

Following on from my presentation this week on Building an Accessible SharePoint System I had an interesting query from one of the attendees regarding accessible CAPTCHA methods.


For those who haven't come across the term, CAPTCHA refers to the method of challenging users with a query that a human could pass but computers cannot (typically represented by images that display distorted text).

The query related directly to accessibility, and specifically how would you achieve CAPTCHA methods in an accessible way?

CAPTCHA themselves have the following statement on their website:

"CAPTCHAs must be accessible. CAPTCHAs based solely on reading text — or other visual-perception tasks — prevent visually impaired users from accessing the protected resource. Such CAPTCHAs may make a site incompatible with Section 508 in the United States. Any implementation of a CAPTCHA should allow blind users to get around the barrier, for example, by permitting users to opt for an audio or sound CAPTCHA"

This of course does not account for users who are both visually impaired and audible impaired (for example, a user who was both blind and deaf).

The solution to this could include a number of workarounds, including mathematical questions ("what is one plus two?") or more "natural language" queries ("What is the colour of the sky on a clear day?") but these could also present other problems.

Firstly generating enough of these prompts to disrupt predicting the responses to them would be problematic. You then have to consider cultural and language barriers, as well as other impairments such as textual or numerical dyslexia.
It's certainly a difficult topic and one that is a challenge to get right without either leaving your site in an inaccessible state or leaving it open to programmatic mis-use.

To find out more about CAPTCHA you can visit the CAPTCHA Website or the CAPTCHA Wikipedia article.