In a proposed rule by the FAA, it argues that software in an Embraer S.A. Model ERJ 190-300 airplane is secure because it's proprietary:
In addition, the operating systems for current airplane systems are usually and historically proprietary. Therefore, they are not as susceptible to corruption from worms, viruses, and other malicious actions as are more-widely used commercial operating systems, such as Microsoft Windows, because access to the design details of these proprietary operating systems is limited to the system developer and airplane integrator. Some systems installed on the Embraer Model ERJ 190-300 airplane will use operating systems that are widely used and commercially available from third-party software suppliers. The security vulnerabilities of these operating systems may be more widely known than are the vulnerabilities of proprietary operating systems that the avionics manufacturers currently use.
Longtime readers will immediately recognize the "security by obscurity" argument. Its main problem is that it's fragile. The information is likely less obscure than you think, and even if it is truly obscure, once it's published you've just lost all your security.
The comment period for this proposed rule is ongoing. If you comment, please be polite -- they're more likely to listen to you.