Auth0 alternatives that are 100% Open Source, no Open-Core
Auth0 is identity infrastructure — someone specifically searching for an open-source alternative usually wants to audit the code that manages credentials, not just cut cost. Of the catalog's 6 Auth0 alternatives, 4 are 100% FOSS, and they aren't interchangeable: each is built for a different case.
Tools that meet this intent
Alternative to Auth0, Okta
Open source SSO and identity management, an alternative to Auth0/Okta.
Alternative to Auth0, Cognito
API-first identity suite, an alternative to Auth0/Cognito.
Alternative to Auth0, Okta
All-in-one multi-tenant identity, an alternative to Auth0/Okta.
Alternative to Auth0, Clerk
Modern, easy-to-use identity, a self-hosted alternative to Clerk/Auth0.
Why being 100% FOSS matters here
Keycloak (Apache-2.0) is the most mature and battle-tested in enterprise environments, with LDAP/Active Directory federation built in — in our own deployment audit it's the only one of the 4 with a verified Docker state. Ory (Apache-2.0) is 100% API-first: it ships no login UI of its own, you design and build the frontend yourself, in exchange for full control of the flow — it has a steeper learning curve for exactly that reason. Zitadel (AGPL-3.0, with Apache-2.0/MIT exceptions in some directories) is built from the ground up for multi-tenancy: isolating each customer's identity without deploying one instance per customer, aimed at B2B SaaS. Logto (MPL-2.0) targets small teams who want something as simple to set up as Clerk but self-hosted, with the most polished admin panel of the group. If your case is offering the identity service itself to third parties (not just using it internally), it's worth reviewing the implications of Zitadel's AGPL-3.0 license for your specific distribution or service case before deciding — this isn't a limitation unique to Zitadel in the open-source ecosystem, but it is the only strong-copyleft license in this group.
What you gain and lose versus the SaaS
None of the 4 is "the Auth0 alternative" in general — they're 4 tools with different design goals. If you need battle-tested enterprise SSO, Keycloak. If you want to build your own login frontend with no constraints, Ory. If your product is a B2B SaaS with multiple customers needing isolated identity, Zitadel. If you're a small team that values a simple setup experience, Logto.