IdentityServer
The most flexible and standards-compliant OpenID Connect and OAuth 2
...We believe that C# is the best DSL, don't bother with trying to express your logic in YAML, XML, or other "languages". In short, if what you want to achieve is doable in .NET Core, it is doable with IdentityServer. You are not forced onto a specific hosting environment or other peoples' servers, you are not forced to use a specific database or geographical region.