Can Windows viruses on an Intel-based Mac damage MacOS X?

The vast majority of viruses are designed to target a particular operating system, and as Windows XP and MacOS X would be running on separate partitions on an Intel-based Mac, viruses written to attack the Windows XP operating system currently are not able to operate "within" MacOS X if installed in the default manner enabled by Boot Camp.

In a MacWorld FAQ regarding Boot Camp, the author also notes that:

If your Windows installation was infected by a virus that tried to delete files on your hard drives, it wouldn't even see your Mac files and they'd be safe. But if you install a program like Mediafour's MacDrive, which gives Windows XP the ability to see Mac volumes, your files could be vulnerable to a virus that deletes files.

In the company FAQ regarding Desktop for Mac, Parallels notes that:

Although computer viruses and similar malicious software [written for Windows] cannot affect MacOS X itself, but the shared folders that are accessible from the Windows guest OS can be corrupted.

Starting with Beta 3036, Parallels Desktop for Mac began supporting "drag and drop" between Windows and MacOS X, as does VMWare Fusion. In an excellent InfoWorld report, the author wisely questions whether the "drag and drop" feature has made virtualization solutions vulnerable to additional security risks.

An earlier InfoWorld report quotes a security expert who first acknowledges that the risk of "crossover" viruses is overinflated, but then he remarks:

Mac and Windows each have their own vulnerabilities, and though I think we'll see a [Mac OS-to-Windows] crossover proof-of-concept before the end of this year, there's just not a lot of value in it for an attacker.

Effectively, given the small number of systems that are capable of running both MacOS X and Windows, this expert thinks that few are likely to bother writing attacks that operate in such a manner. He also mentions the possibility of an attack that could damage the partition table, potentially causing problems for any operating systems installed on the system.

As also mentioned previously, it also is possible that the Extensible Firmware Interface (EFI) used by the Intel-based Macs could be attacked. Fortunately, a C|Net article quotes a variety of security experts who conclude that such attacks on "lower-level system software. . . are rare". In general, there are few with the level of programming sophistication needed to write an EFI attack.

In addition, C|Net provides a basic Windows Security on Mac primer that you may find helpful.