But note that's only due to popularity. Socially engineering your way into a user running an executable means that executable will simply run with user privs. No trickery or hacking required, no OS holes. And that will mean that the executable will have full access to do everything a user could do, which will effectively certainly include sending a new encryption key over the network, and encrypting every file that user can get a hold of.

(One of the little problems with the UNIX-style user permissions is that it is designed to defend the OS, not the user. Sure, that little executable may not be able to corrupt "the system", which may amount to 5 or 10 GBs of easily-replaced code, but it will have its way with the 2TB of the single user's media files.)

The only faint defense Linux/UNIX can claim is the slightly higher probability that you'll be on a checkpointing file system and can roll back, and I say only "slightly" because they still aren't very popular yet compared to conventional file systems.