I've used [Keycloak](https://www.keycloak.org/) in the past for "open-source Auth0" -- though I'm not sure it has ever described itself that way.
Keycloak ended up being quite extensible and powerful, but the UI and data model both sometimes made things more difficult than they had to be... this could be an interesting project to look at.
One bonus (for us) for Keycloak was that it was JVM-based, meaning it was easier to integrate our existing JVM libraries. Though its use of Hibernate was frustrating at times, heh
Keycloak ended up being quite extensible and powerful, but the UI and data model both sometimes made things more difficult than they had to be... this could be an interesting project to look at.
One bonus (for us) for Keycloak was that it was JVM-based, meaning it was easier to integrate our existing JVM libraries. Though its use of Hibernate was frustrating at times, heh