@julian Great work, it works for me as well.

@Paolo exactly you are right. I have the same purpose as yours. It's possible to persist the session in MongoDB and share between two node apps. I had tested this ways as well. However, it's quite more difficult than sso solutions, since you need to implement in your node app with all NodeBB User relevant stuff, maybe still groups and others. I tried to do that and it worked already. Now I found it's better to enable sso login and disable nodeBB local login.
For a simple oauth2 provider, you can find an example in the oauth2orize repository: oauth2 provider
For the client side, all you want is the plugin which Julian had implemented.