Hacking operating systems (OSs) is a preferred method of the bad guys. OSs
comprise a large portion of hacker attacks simply because every computer
has one and so many well-known exploits can be used against them.
Occasionally, some operating systems that are more secure out of the box —
such as Novell NetWare and the flavors of BSD UNIX — are attacked, and
vulnerabilities turn up. But hackers prefer attacking operating systems like
Windows and Linux because they are widely used and better known for their
vulnerabilities.
Here are some examples of attacks on operating systems:
Exploiting specific protocol implementations
Attacking built-in authentication systems
Breaking file-system security
Cracking passwords and encryption mechanisms