How can I attach security bars over EIFS stucco?
You have a couple options, depending on the level of pry resistance you require. 


Pilot for screws into the wall sheathing. Use at least a #10 wood screw, and pilot about half the shank diameter into the wood, then the shank diameter through the stucco (to prevent spalling and cracks). With long enough screws you'll have good grab into 1/2" wall sheathing, which should be OSB or plywood.  It is possible that you have fiberboard or gypsum sheathing, in which case this won't work at all. If your bars have two screws at each corner this will provide fairly solid holding power. If there's only one screw, I'd look at #2. 
Find the studs. They will probably be on 16" intervals from one end and alongside each window or door. You might have the best luck searching from the inside using a stud finder or a small drill bit. Use wood screws that penetrate at least 1-1/2". Pilot through the stucco and a short distance into the stud to guide the screw and prevent glancing misses. 
Use a more modern means of security. An electronic system with glass break sensors might be more suitable. Screws can be removed and bars can be cut. 


Notes:


The foam behind the stucco will compress under heavy forces, resulting in damage to the stucco. Do not run the screws in beyond slightly snug. 
Drilling through the stucco compromises its weather resistance. Consider dabbing silicone or urethane caulk behind the hardware as you install it to seal those penetrations. 
Repairing stucco (beyond small screw holes) is an art form, and getting a good texture and color match can be difficult. Do as little damage as possible. 
Hitting studs usually requires custom sizing or adjustability. This may be a problem when buying off the shelf.