mCaptcha

mCaptcha is a privacy-focused CAPTCHA system that uses proof of work instead of image puzzles. The core software is free and open source, so sites can run it themselves.

A visitor's browser does a small computation before a form is accepted. The cost is set so that automated attacks become expensive to run at scale. Because no image puzzles are shown, the check is mostly silent for people. The system does not track or profile visitors, and it does not use IP addresses for rate limiting.

• Uses proof of work instead of image puzzles
• No IP addresses used for rate limiting
• Compatible with reCAPTCHA and hCaptcha APIs
• Client libraries under permissive free licenses
• Core software under the AGPL

mCaptcha suits privacy-minded website owners, self-hosters and developers who want a CAPTCHA they control. It also fits sites with users behind shared networks, since the check does not depend on IP addresses.

mCaptcha is open source, and the source is on GitHub. Its core is licensed under the AGPL, which requires modified versions run as a network service to share their changes. Check the docs before you move from another CAPTCHA, since the API is designed to be a drop-in replacement.