The profile of malicious code on the Internet is changing with porn diallers and Trojan horses becoming more serious problems. A study on the malicious code blocked last year by managed services firm MessageLabs finds the spread of Trojan horses is becoming more organised.

From recording Trojans sporadically, MessageLabs is now intercepting 40-50 Trojans at a time. These are systematic attempts to infect victim's machines, it says.

Sex diallers are another growing problem. These change the number used by dial up connections to expensive premium rate lines. Such programs, which are regularly modified by their creators and pose legal problems for AV firms. By adding detection for the programs (which don't load without user interaction) the vendors could be accused of restraining trade. One man's malware is another's useful utility.

MessageLabs gets around this by filtering (quarantining) such traffic rather than deleting it. During 2001, the vast majority of viruses blocked by MessageLabs were Word macro viruses but few achieved any prominence. Built by s'kiddies and using the same infection techniques time and again most AV tools with heuristic functions easily detect such nasties, said Alex Shipp, a senior anti-virus technician at MessageLabs.

Word macros also pose a lesser risk because Office 2000 requires macro code to be signed, so that the spread of such viruses is becoming more constrained, he added.

Shipp is slightly more concerned about the risks posed by script viruses which, as the Kournikova worm proved, can spread rapidly if rates of infection reach a critical mass. Again, virus writers are failing to exercise much "imagination", and this is keeping the problem under control, MessageLabs believes.

Because there are so many ways to write

Windows executable viruses form the main danger going forward, as there are so many ways to write them, Shipp said in a presentation at the Virus Bulletin conference in New Orleans last week.

Add in the increased use of exploits used by spammers in virus writing and the war between VXers and AV vendors is moving to a new battleground, he says.