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Casdoor - FOSS IAM SSO
(github.com)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Neither, it's a method to allow users to login to your app/site, without needing a specific registration there. It does not replace Bitwarden, that is client side. This is server side.
EG you set this up and then on your app login, you say 'hey, login with facebook'. Casdoor handles the authorization and authentication to Facebook, returns a code to your site 'Yes thats good' or 'No that's not'. Then you handle accordingly