Jimoblak, let me take this point by point. I suspect you and I are in close agreement on this topic, I just have a little issue with some of your statements.
First, your point is well-taken that there is user-susceptibility vs. machine- or OS-susceptibility. Mac users are just as susceptible, perhaps even more so, to social engineering stunts as Windows users. Like the report of the guy who executed malicious Applescript code downloaded from Limewire because it had the Microsoft Word icon on it. Ouch. There is not an operating system out there that can prevent a user from deliberately, willfully launching malicious code. Yes, a sucker is born every milli-second.
Second, your point about only one virus or hacker being required to ruin your whole day is also very valid. I don't care if there are 5 or 5 million viruses in existence, so long as they aren't on my systems. And just one, even if it were the only one in existence, would be too many on my system. Agreed.
Here's a statement you made in your first post on this topic:
[highlight]In terms of susceptibility, Mac OSX is just as susceptible as Windows to viruses. Every operating system can be exploited in one way or the other. [/highlight]
And in the follow-up post:
[highlight]Being susceptible has nothing to do with market share or the number of known exploits for a particular system. [/highlight]
If someone asked you to devise a scale to measure or assess 'susceptibility', would you answer with a binary formula? In other words, a system is either susceptible or its not, and that's how everything will be judged under your scale? As you point out, every operating system can be exploited in one way or another, so everything would end up at the upper end of your scale, 'Susceptible'. Not a terribly useful scale if it doesn't distinguish between any systems at all, now is it?
Wouldn't it make more sense to rate systems by the number of known exploits and severity of those exploits? Yeah, it only takes one to ruin your whole day, but if we are assessing one OS vs. another, isn't this a more useful way to discuss the situation than a simple statement that all systems are vulnerable, therefore all systems are equally vulnerable? That's why I ask about references from CERT or Apple's Knowledgebase, because those places will include the severity of the exploits.
Getting back to the original question, which I don't think this thread has done a very good job of answering, why is the Mac OS less susceptible to viruses? Others have addressed Mac OS <9, along with a great remark about telnetting into a rock, which is what Mac OS 9 and earlier were. Network rocks, out of the box. Sure, you could open them up very easily by clicking a box in the FileSharing control panel, but the default settings were pretty solid.
Mac OS X, however, is a whole different ball game compared to OS < 9. I think it is important to distinguish between auto-execute and other types of vulnerabilities, for the practical reason that auto-execute stuff is what allows these Windows worms to circle the globe overnight and infect millions of machines. Anything that can't auto-execute is simply not going to be able to cover that much territory and will move way, way slower.
Here's the general answer to the question, I believe:
OS X has different privileges (modes) for different parts of the operating system, which means it is tricky to get a piece of code to execute on a box without user intervention. OS X has an idea of ownership, which makes it difficult for code 'owned' by a user to touch files 'owned' by the system, without user intervention. OS X in a managed environment, i.e., most corporate environments, is set up such that the actual user(s) of the machine aren't admins, so they can't install new software, or can't install it in the system-space. Only in their own sandbox, which greatly limits the ability of that code to do damage.
One thing I don't understand is why there isn't more malicious Applescript out there. It really should be trivial to write an Applescript that sends a copy of itself to all entries of the users address book. It would just be one email with a cc or bcc of everyone in the address book, so there wouldn't be lots of disk activity to tip off the user, if all the addresses were valid, there wouldn't be any bounces to tip off the user. Most Mac users have other Mac users, at least a few, in their address book. Attach the applescript to the outgoing email with the title "OS X Startup Optimizer script from OSXHints.com", and at least half your recipients would run it, even if they had to authenticate to do it. Okay, so there's an opportunity for a virus writer, but that's a social exploit, not an OS susceptibility.
Anyway, as I say, I think we agree very closely, I just like to make finer distinctions in the area of vulnerabilities, because I believe there are practical reasons to do so.
Thanks for your thoughts on this topic, it's been an interesting read.