A few points about security in general (about which I know something), not specifically about aviation:
- Security systems get broken even when the designers and other experts swear it just physically can't happen. I'm not suggesting other posters are wrong to say aircraft control systems can't be hacked, I'm just saying sometimes "impossible" hacks happen.
- As Bruce Schneier put it (he's one of the leading industry experts on security): "Any security expert can devise a security system so secure that he or she can't conceive of any way of breaking it". But other people will.
- For this reason, the best security systems are not secret: the details of architecture, design and procedures are open and subject to scrutiny. That way it's more likely that the good guys find the problems before the bad guys do. Because, assuredly the bad guys WILL find the problems, and it's much worse if they find them before you do. The security should rely on secrecy of keys and passwords (which can be changed), not on secrecy of design (which can't easily be changed).
- This is one of consequences of the Digital Millenium Copyright Act (DMCA). It criminalises attempting to hack or even investigate commercial cryptography systems. Of course, it does nothing to stop the bad guys (it doesn't even slow them down), but makes it much less likely that security vulnerabilities will be found by anyone else (and fixed).
- A number of security systems in widespread use today were hacked very quickly, and arguably with more public scrutiny early on they might have been enhanced and corrected before launch. Two examples are the CSS encryption system used on DVDs and the Mifare contactless card used in numerous public transport systems (such as the London Oyster card).
As I understand it, most avionic systems and interfaces are open and public. From a security point of view, that's good, not bad. If people are worrying publicly about ACARS and ADS-B, it's much more likely any problems get found and fixed, e.g. by manual checks and balances in the cockpit.