If you're trying to get data from a secure network to a less secure one, there's a device called a data diode which can't be hacked traditionally because it can only send signals outward and not inward. Think of a fiber optic cable where you only have a transmitter on one side and a receiver on the other, or an AM radio -- you can't hack the radio station no matter how you turn the dial on your am radio because the info only moves from the station to your radio. Contrast with a 2-way link into a secure network from a third party.