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Date: 2011-12-23 12:58 am (UTC)Windows NT/2000/XP/Vista/7/Server, aka the Windows NT family, are vulnerable not because they are a single-user OS. NT never was a single-user OS. The roots of Windows NT lie in Digital's very secure, very multi-user VMS operating system (look up Dave Cutler and Microsoft when you get a chance). Windows/NT is vulnerable to attack because of an architectural change made in NT 4.0. Specifically, Microsoft moved disk and graphics I/O processing from protected kernel space to unprotected user space. The reason is performance: moving these I/O functions to user space drastically reduced security context switching which in turn drastically improved overall performance of the OS. The problem with this is that low-level device I/O requires low-level device access privileges. These functions provided that access in a way that bypasses the CPU security layers. Gaping big hole.
OS X isn't based on System V UNIX. OS X's kernel history lies along the BSD UNIX path. Originally BSD UNIX plus Mach in NeXTStep and OpenStep, then a mix of FreeBSD and NetBSD plus Mach for early versions of what became OS X's kernel, and eventually just FreeBSD plus Mach since version 10.4. OS X isn't based on UNIX, it *is* UNIX, just the BSD flavor rather than the System V flavor.
The Linux kernel was inspired by MINIX, not System V or BSD. Early development was on MINIX and the first Linux file system was the MINIX file system. This might sound like picking nits, but ask a systems-level developer and he'll rant at you about the differences between Linux and real UNIX. If you're (un)lucky you may also get a rant about the differences between System V and BSD. :)