Now you're into the highly classified world of intelligence sharing, and I would just be guessing.
But one thing that is missing—this is a side note to this cyber war thing and the cyber threat, and I'm not trying to downplay the problem here—is that we have numerous examples historically in which nations tacitly agree out of their own self-interest, in the context of potential warfare between them, not to do certain things. Given that modern societies and increasingly even developing societies—all of us—are vulnerable to the cyber world, potentially this is an area we need to start talking about or tacitly to recognize that it is something wherein...all our interests lead us away from doing this.
Now, that doesn't account for the potential terrorist attack and the non-state actors involved, about which we do have to be concerned. But I'm a bit of a skeptic about the emphasis we should place and the amount of investment outside of normal security requirements that we need to make in these areas.