AFAIK, the BSD kernel that OSX stole is (was)just as secure as the Linux kernel. The reason that the Linux kernel is secure is that it doesn't allow arbitrary execution of code, which only leaves "L337 low-level wizardry" and "user stupidity/phishing for idiots" exploits. However, where Windows and OSX fail is that they've engineered and added all kinds of big brother-ware/back-door/"ET Phone Home" functionality in which hackers then discover and exploit.