Two days ago the Italian exchange told nano core devs that their hot wallet was hacked by one of their users.
While BitGrail asserts they discovered they hack just that morning it seems the theft was ongoing for months before that.
Firano, the owner of BitGrail, suggested to fork the chain and just restore the money. This option was rejected by the author of nano and I sincerely doubt it would be easy since nano it's not a linear blockchain but a DAG.
While Firano implies the hack was due to a bug in the node software, it’s likely that the exchange itself has faulty code.