What are two main types of security in Linux?

Answer 1

My first guess is preventive and reactive.

A longer answer includes lists of antivirus, antispam, intrusion countermeasure, proper network settings and OS updates. Many of them can be found in Linux blogs, SANS security white papers, Cnet free downloads and DriverGuide.com

Answer 2

If you're in the government, and you really need to know this information, each vendor has to apply for their own certification, so you should be asking the vendor. It can be extremely secure, or it can be wide open.

A non-networked Linux system is probably around a C2, but I don't think anyone has actually forked over the money required to have it certified.

Answer 3

Linux's kernel is generally very secure. Root permissions are necessary to do many critical tasks on the operating system, and by default this account cannot be logged in to. Since it is open source any vulnerabilities can be discovered and patched quickly, unlike in proprietary operating systems.