While it hasn't infected a lot of computers yet, Mac Defender has been making a lot of noise as one of the first major Mac security threats. Here's a look at how to protect yourself from it and avoid it altogether.
Search Before You Download
Mac Defender — now also masquerading as MacGuard, MacProtector and MacSecurity — is pretty easy to avoid. First of all, if you see any offers for either of these applications — don't download them. This is probably fairly obvious, but for the less Mac-savvy who aren't used to virus and malware threats, it's an important reminder. If you don't know what the software is, do a web search for its name and find out whether or not it's malicious. If it is, you're probably not the first one to find out and report it online.
Disable the Automatic Opening of Downloaded Files
If you're browsing the web with Safari you'll want to disable the option that automatically opens your downloads. To do this, go to the Safari menu, choose Preferences, and in the General tab look for an item at the bottom that reads "Open safe files after downloading". This is checked by default. Just uncheck it and you won't have to worry about accidentally opening and installing Mac Defender (or whatever pseudonym it has chosen).
Finally, it's worth keeping an eye on this Apple support doc. While there isn't much information there at the moment, it seems to be a placeholder for Apple's official instructions on the Mac Defender problem.
These are both pretty easy things to do, and chances are most Lifehacker readers are savvy enough to know about them. If you know someone who's got a Mac and might be at risk, be sure to forward this information along.