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But, as Melissa, the IloveYou, and other widespread infections on millions of Windows machines have proven, people often tend to click before they look. This preying on impatience and faith is key to the security issue with Macs that does point to a danger. Reported by German news web site Heise Online, and originally discovered by a German graduate student, Michael Lehn, this security hole, labeled “Mac OS X File Association Meta Data Shell Script Execution,” has been deemed “extremely critical” by security firm Secunia.

Basically, it relies on the way Apple web browser Safari handles downloaded files. By default, Safari automatically opens “safe” files without asking for user confirmation. Lehn discovered that, although Safari usually requires confirmation before it opens an application or shell script, it won’t recognize a script that doesn’t have certain code in it. Eric Bangerman, who covers Apple issues for the web site Ars Technica, says that this hole could allow someone to create a disguised script that could wreak havoc on an Apple machine, deleting a directory or worse.

Yet the hole actually doesn’t offer a malicious hacker much else. Bangerman notes that most criminal hackers want financial gain. As a result, many Trojan horses or worms either turn users’ machines into “zombies” for sending spam e-mail, or install keyloggers, programs that log all keystrokes made on a computer and steal that information.

Mac OS X makes these options highly unlikely, if not impossible, Bangerman says. In the case of keyloggers and other malicious applications, Mac OS X warns users when an application is starting up for the first time. Again, many users might just click through this warning, but at least it’s there. And Apple designed Mac OS X so that it is difficult to run as a root user (that is, with full access to operating system settings), which would be required to turn a Mac into a zombie.

Still, it’s a good idea for Mac OS X users to take some defensive action. Even though no cases using this Safari hole have been spotted, it’s no secret that the hole exists. Fortunately, the best solution is the easiest. Users can go to Safari’s preferences and turn off the “safe files” option. Or they can use Firefox, Camino, Opera, or another alternative Web browser that doesn’t have an auto-open feature. For further security, one can run the ClamAV anti-virus application or Unsanity LLC’s Paranoid Android 1.3 – both are free.

Of course the best solution would be for Apple to close the hole in Safari – a task that Bangerman says should be easy to do.

(Apple declined several requests for comment on this story.)

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