visualCaptcha is a configurable captcha solution, focusing on accessibility & simplicity, whilst maintaining security. It's also localizable and free, open-source, and we encourage customizations per site.
It also supports mobile, retina devices, and has an innovative accessibility solution. visualCaptcha supports many back-end and front-end languages/frameworks/libraries, which you can look into in more on its github page: https://github.com/emotionLoop/visualCaptcha
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