OpenAuth is a universal, self-hosted, standards-based auth provider that can be deployed as a standalone service or embedded into an existing application. It works with any framework or platform and implements the OAuth 2.0 spec, allowing any OAuth client to use it. Unlike other solutions, OpenAuth is a centralized auth server that runs on your own infrastructure and can be used to authenticate all applications. It does not attempt to solve user management, but instead invokes a callback for user lookup/creation logic. OpenAuth also has a themeable UI and is supported by SST for easy deployment to AWS or Cloudflare.
https://openauth.js.org/