I'm fully agree the threat model is different for small vs huge system.but it is different from the "service" point of view..
this is not true because attacks on centralized application with a goal to access the data of all users will focus on on generic issues rather single accounts (e.g. brute force attack).. such attacks are more frequent on centralized services and less common on small instances.
MFA doesn't protect against application issues it only protects against attacks on a single account credentials e.g. "password spraying" or "credential stuffing" and this protection is completely unrelated where you account is hosted at home or on a big cloud instance.
it works for for some degree.. for Joplin server it might work somewhat but this approach fails for Nextcloud and other services where you want to share data outside of your bubble.. VPN adds more complexity and many operational issues.. it works if you live in good old 20th century but fails in a today always connected world..
I'm self-hosting JoplinServer, Nextcloud, Jellifin, Keycloak, Zitadel and other services because I can.. likely it would cost less effort and maybe less money to use hosted service but I don't like vendor lock-in.. nobody knows how long you are committed to provide your cloud service..despite the fact nobody offers all this services.. and managing multiple hosting providers is complex as well..
There are marketing reasons to limit features like MFA to a paid cloud instance but this reasons are not security or requirements.. the only legitimization for such cut on a OSS application is the desire to sell "premium" features of the hosted service..If this is the reason I can live with it and somewhat understand it.. but please speak clear language - if you are behind the money OK bad for the users but good for the community. If you are a good open source guy - tell us what you need to add this high value feature to self-hosted Joplin Server.
PS:
LDAP is is related to SSO but it doesn't address MFA (not part of the protocol) neither it protects against before mentioned attacks.. it makes some things easier but still doesn't provide modern account protection..
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