Open.IdentityServer provides a free, community‑run fork of the retired IdentityServer4
IdentityServer4 has reached end‑of‑life, so Rock Solid Knowledge forked the code into Open.IdentityServer, a free, community‑maintained OpenID Connect and OAuth 2.0 framework for ASP.NET Core. .NET teams can now self‑host a fully supported identity solution without paying for Duende licenses.