Of Trojans and Viruses

Of Trojans and Viruses
Mac Tip #155/30-June-2004

In a recent Tip I mentioned trojans and viruses and that your anti-virus software won’t necessarily detect trojans.


A virus is a piece of software deliberately written to do harm to your computer. It seems that for whatever reason virus writers don’t generally bother with creating viruses for the Mac. That’s not to say that a new Mac virus won’t pop up tomorrow though.

A virus usually contains code which helps it spread. For example, on Windows computers it’s common for a virus to identify email addresses stored on the computer and arrange to get itself sent to them. These days it often also arranges to claim it’s coming from another of the addresses it finds.

The outcome of that is that if Joan, a Windows user, has your email address on her computer and also has my address then you may receive virus-bearing emails from her which claim to come from me (or vice versa).

In the old days some systems receiving virus-bearing emails would send a message to the sender warning about the virus. That was a handy service at that time but now it just confuses people and clogs the email system.

Next time you receive a message claiming that you emailed someone a virus just ignore it. The problem email almost certainly didn’t come from you.


A trojan is a horse of a different colour. Harking back to the story of the Trojan Horse, a wooden gift horse concealing soldiers, this nasty disguises itself as something useful and desirable. Inside though it contains harmful coding. A trojan doesn’t usually replicate or spread in the way a virus does.

While a virus will simply “arrive” in an email or on an infected disc, you generally need to go and get a trojan, or someone will deliberately send it to you.

In April 2004 an anti-virus company claimed it had found the first malicious trojan for Macs.

This trojan looked like a music file in the MP3 format. If you double-clicked the file to play the music the hidden code could carry out some actions on your Mac. In this case it would display a warning, but in theory it could delete files.

It turned out that this trojan was simply theoretical. It was a demonstration of a possible threat.

Then in May 2004 a Mac magazine reader downloaded a file from a known disreputable source. He “thought it was 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.” (Quoted from TidBITS #730/17-May-04)

Apple’s latest Security Update now brings up an alert on your screen if you open certain files. It should no longer be the case that a trojan can just wipe out all your files. That’s why it’s important to install any Security Updates available to you. There’s more information at Apple’s website.

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