ALTCHA is free, open-source CAPTCHA alternative that protects your website from spam and abuse.
It uses a proof-of-work mechanism to protect your website, APIs, and online services from spam and unwanted content. Unlike other CAPTCHA solutions, ALTCHA is free, open-source and self-hosted, does not use cookies nor fingerprinting, does not track users, and is fully compliant with GDPR.
Refer to the documentation.