A few weeks after the hullabaloo surrounding Intego's press release about a technique that could be used to create a Trojan horse that looked like an MP3 file (see "Mac OS X Trojan Technique: Beware Geeks Bearing Gifts" in TidBITS-726), a real Mac OS X Trojan horse has been reported to Macworld UK. The Trojan horse, which purports to be a Web installer for Microsoft Word 2004, does not use the technique previously revealed, but it's decidedly malicious. If you are foolish enough to run it, it deletes your entire Home folder.
In the somewhat confused article, Macworld UK says that the reader who reported it to them downloaded it "from LimeWire." (LimeWire is actually client software for the Gnutella file sharing network.) This reader, proving that common sense isn't as common as would be ideal, somehow thought that the file must have been a public beta of the next version of Microsoft Word, so he downloaded it, noticed that the icon "looked genuine and trustworthy" and double-clicked it, only to discover that it had instead deleted his Home folder.
Our searches of the Gnutella network using Acquisition (a truly elegant Macintosh program, particularly in contrast to the brutish LimeWire, which we also used to search), came up empty. Since the IP numbers of those sharing files on the Gnutella network are readily available, it's highly likely that whoever initially seeded the Gnutella network removed the Trojan horse to avoid further detection, and since detection is easy, it's relatively unlikely that even bozos would knowingly share such a malicious program.
Macworld UK initially chose not to reveal the technique used, but Intego, showing a continued extreme lack of judgment, promptly issued a press release linking to further information that explained almost exactly how to create a similar Trojan horse. Macworld UK then republished Intego's information, and many other sites jumped on it as well. As best I can tell, the argument for publishing the technique is that if people know how it's done, they can better identify and avoid such Trojan horses in the future. That's specious at best, since a Trojan horse merely must deceive a user long enough for that person to double-click; knowing what language it's written in is irrelevant. All that publicizing the technique does is increase the number of people (large though it may have already been in this case) who have the capability to create such a Trojan horse. The cynical are already wondering if Intego's publicity of the previous Trojan technique may have played a role in the creation of this one. If Trojan horse reports continue to roll in, the fault will lie with Intego and everyone else who published the instructions.
Suffice to say that the technique is extremely simple; this Trojan horse merely preys on gullibility and cupidity to sucker people into launching (arguably, it's a bit of digital Darwinism at work). It's worth noting that this Trojan also doesn't exploit any weaknesses in Mac OS X; it's just a deceptively named program that deletes files, and there's no foolproof way to prevent deceptively named malicious software on any platform. No anti-virus software is necessary to detect this Trojan, and it does not replicate itself. As long as you don't download applications from untrustworthy sources, you have nothing to worry about, particularly if you maintain regular backups.