As long as the computer is in its own simulated world, with no input from the outside world, we're almost certainly safe. It cannot model the real world.

Note: given really really large computational resources, an AI can always "break out by breaking in"; generate some physical laws ordered by complexity, look what sort of intelligent life arises in those cosmologies, craft an attack that works against it on the assumption that it's running the AI in a box, repeat for the hundred simplest cosmologies. This potentially needs a lot of computing power, but it might take very little depending on how determined our minds are by our physics.