Depends how you define fault. The way I see it, it was both Sony and the hackers' fault, because they were both responsible for the events that took place.
Another way to see it is that if the hackers didn't attempt to hack in the first place, there would have been no problem, therefore it is only the hackers' fault.
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_foolmo_
'Illegal activities is a slight misnomer, most of it is not related to material that is actually illegal.' - nintendogrl1 (Mod)