Could a terrorist hack the ILS?
There are numerous unclassified papers talking about LOC, GS, MLS and other navaid jamming. There are also instances of that happening, both intentionally and unintentionally.  The issue with consumer electronics is made worse is that local consumer electronics on the plane can have local oscillators in the receivers.  They are small signals, but they are close to the antenna, hence the greater risk. 

Jamming LOC (lateral ILS guidance) and VOR are similar. GS interference is a bit more difficult to provide false readings that make sense to a pilot, when the transmitter is off airport.

The reality is that even GPS can be jammed, faked with stronger signals, etc. Numerous companies make GPS-GNSS simulators, and it is not rocket science to integrate those into jamming systems.  Some of the countermeasures are documented in open literature, and other countermeasures and methods are not.  GPS jamming becomes an issue with various weapons delivery systems, and creating effective countermeasure packages with limited size-weight and power (SWAP) is a constant challenge.

I concur with @STWilson, that if faced with the possibility of interference, it would be good practice to monitor your approach with GPS, NDB, OM and other navaids.  For example, one can always start a timer crossing the OM, even if doing an ILS. Certainly in a war or terrorism situation, additional cross-checks are good investments of effort.