What data about me could Lemmy hackers steal?
Theoretically speaking of course ;)
If my home instance gets hacked, what’s the worst case scenario for my personal data?
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Nothing. Everything your instance has is your IP address (mostly useless) and password hash (also mostly useless). Everything you have here is public. Maybe except your settings, like light/dark mode.
Password hashes are only useless if you have a good password to begin with.
If not, they can likely get your actual password from it if you re-use passwords etc.