The best open source alternative to FusionAuth is Keycloak, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to FusionAuth and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to FusionAuth are Ory (Free, Open Source), Gluu Server (Freemium, Open Source), openNAC (Free, Open Source) and FreeRadius (Free, Open Source).
Ory builds cloud-native authentication (login), authorization (permission), delegation (OAuth2, OpenID Connect), and user management software. You can deploy what you need yourself or use the Ory Cloud Platform and get started in minutes.
FreeRADIUS is responsible for authenticating one third of all users on the Internet. Welcome to the FreeRADIUS project, the open source implementation of RADIUS, an IETF protocol for AAA (Authorisation, Authentication, and Accounting).
authentik is an open-source identity provider focused on flexibility and versatility. - Easy to Use: Identity made easy. authentik makes single-sign on (SSO), user enrollment, and access control simple.