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.
To their credit, Macworld UK chose not to reveal the technique used, which we hope will reduce the number of copycats. Suffice to say that the technique is 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.