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Yes, but I expect that upvu has stored its keys safely

That is exactly what crossed my mind.
If users as experienced as people behind @upvu could become a victim, then safety of our keys on steem is a real question ...

Yes, and I think we will see more attacks going forward

I also assume that. But if that is the case, I find it even more worrying!
Because then the key must be determinable from other sources...