about 'the kids', Mathieson's rhetoric glosses over a charged ethical and legal debate. It is tempting to wonder if the leading malware authors are lying, whether they do in fact circulate their worms on the sly, obsessed with a desire to see if they really work. While security officials say that may occasionally happen, they also say the top virus writers are quite likely telling the truth.

'If you're writing important virus code, you're probably well trained,' says David Perry, global director of education for Trend Micro, an anti-virus company. 'You know a number of tricks to write good code, but you don't want to go to prison. You have an income and stuff. It takes someone unaware of the consequences to release a virus.'

But worm authors are hardly absolved of blame. By putting their code freely on the web, virus writers essentially dangle temptation in front of every disgruntled teenager who goes online looking for a way to rebel. A cynic might say that malware authors rely on clueless script kiddies the same way that a drug dealer uses 13-year-olds to carry illegal goods, passing the liability off to a hapless mule.

Some academics have pondered whether virus authors could be charged under conspiracy laws. Creating a virus, they theorise, might be considered a form of abetting a crime by providing materials. Ken Dunham, the head of 'malicious code intelligence' for iDefense, a computer security company, notes that there are certainly many examples of virus authors assisting newcomers. He has been in chat- rooms, he says, 'where I can see people saying, "How can I find vulnerable hosts?"And another guy says, "Oh, go here, you can use this tool." They're helping each other out'.

There are virus writers who appreciate these complexities, but they are certain that the viruses they write count as protected speech. They insist they have a right to explore their interests. Indeed, a number of them say they are making the world a better place, because they openly expose the weaknesses of computer systems. When Philet0ast3r or Mario or Mathieson finishes a new virus, they say, they will immediately email a copy of it to anti-virus companies. That way, they explained, the companies can program their software to recognise and delete the virus should some script kiddie ever release it into the wild. This is further proof that they mean no harm with their hobby, as Mathieson pointed out. On the contrary, he said, their virus writing strengthens the 'immune system' of the internet.

These moral nuances fall apart in the case of virus authors who are themselves willing to release worms into the wild. They're more rare, for obvious reasons. Usually, they are in countries where the police are less concerned with software crimes. One such author is Melhacker, a young man who reportedly lives in Malaysia and has expressed sympathy for Osama bin Laden. Anti-virus companies have linked him to the development of several worms, including one that claims to come from the 'al-Qaeda network'. Before the Iraq war, he told a computer magazine that he would release a virulent worm if the United States attacked Iraq, a threat that proved hollow. When I emailed him, he described his favourite type of worm payload: 'Stolen information from other people.' He won't say which of his viruses he has spread and refuses to comment on his connection to the al-Qaeda worm. But in December on Indovirus.net, a discussion board for virus writers, Melhacker urged other writers to 'try to make it in the wild' and to release their viruses in cybercafes, presumably to avoid detection. He also told them to stop sending in their work to anti-virus companies.

Mathieson wrote a critical response, arguing that a good virus writer should not need to spread his work. Virus authors are, in fact, sometimes quite aggrieved when someone puts a dangerous worm into circulation, because it can cause a backlash that hurts the entire virus community. When the Melissa virus raged out of control in 1999, many internet service providers shut down the websites of malware creators. Virus writers stormed online to pillory the Melissa author for turning his creation loose. 'We don't need any more grief,' one wrote.

If you ask cyberpolice and security experts about their greatest fears, they are not the traditional virus writers, like Mario or Philet0ast3r. For better or worse, those authors are a known quantity. What keeps anti-virus people awake at night these days is an entirely new threat: worms created for explicit criminal purposes.

These began to emerge last year. Sobig in particular alarmed virus researchers. It was released six separate times throughout 2003, and each time the worm was programmed to shut itself off permanently after a few days or weeks. Every time the worm appeared anew, it had been altered in a way that suggested a single author had been tinkering with it, observing its behaviour in the wild, then killing off his creation to prepare a new and more insidious version. 'It was a set of very well-controlled experiments,' says Mikko Hypponen, the director of anti-virus research at F-Secure, a computer security company. 'The code is high quality. It's been tested well. It really works in the real world.'

By the time the latest variant, Sobig.F, appeared in August, the worm was programmed to install a back door that would allow the author to assume control of the victim's computer. To what purpose? Experts say its author has used the captured machines to send spam and might also be stealing financial information from the victims' computers.

No one knows who wrote Sobig. The writers of this new class of worm leave none of the traces of their identities that malware authors traditionally include in their code, like their screen names or 'greetz', shout-out hellos to their cyberfriends. Because criminal authors actively spread their creations, they are cautious about tipping their hand. 'The FBI is out for the Sobig guy with both claws, and they want to make an example of him,' David Perry notes. 'He's not going to mouth off.

Dunham of iDefense says his online research has turned up 'anecdotal evidence' that the Sobig author comes from Russia or elsewhere in Europe. Others suspect China or other parts of Asia. It seems unlikely that Sobig came from the US, because American police forces have been the most proactive of any worldwide in hunting those who spread malware. Many experts believe the Sobig author will release a new variant sometime this year.

Sobig was not alone. A variant of the Mimail worm, which appeared last spring, would install a fake pop-up screen on a computer pretending to be from PayPal, an online e-commerce firm. It would claim that PayPal had lost the victim's credit card or banking details and ask him to type them in again. When he did, the worm would forward the information to the worm's still-unknown author. Another worm, called Bugbear.B, was programmed to employ sophisticated password-guessing strategies at banks to steal personal information. 'It was specifically designed to target financial institutions,' said Vincent Weafer, senior director of Symantec.

The era of the stealth worm is upon us. None of these pieces of malware was destructive or designed to cripple the internet with too much traffic. On the contrary, they were designed to be unobtrusive, the better to secretly harvest data. Five years ago, the biggest danger was the 'Chernobyl' virus, which deleted your hard drive. But the prevalence of hard-drive-destroying viruses has steadily declined to almost zero. Malware authors have learned a lesson that biologists have long known: the best way for a virus to spread is to ensure its host remains alive.

'It's like comparing Ebola to Aids,' says Joe Wells, a founder of WildList, a long-established virus-tracking group. 'They both do the same thing. Except one does it in three days and the other lingers and lingers. But which is worse? The ones that linger are the ones that spread the most.' The long years of experimentation have served as a sort of Darwinian evolutionary contest, in which virus writers have gradually worked out the best strategies for survival.

Given the pace of virus development, we are probably going to see even nastier criminal attacks in the future. Some academics have predicted the rise of 'cryptoviruses', malware that invades your computer and encrypts all your files, making them unreadable. 'The only way to get the data back will be to pay a ransom,' says Stuart Schechter, a doctoral candidate in computer security at Harvard.

This new age of criminal viruses puts traditional malware authors in a politically precarious spot. Police forces are under more pressure than ever to take any worm seriously, regardless of the motivations of the author.

A young Spaniard named Antonio discovered that last year. He is a quiet 23-year-old computer professional who lives near Madrid. Last August, he read about the Blaster worm and how it exploited a Microsoft flaw. He became intrigued and after poking around on a few virus sites, found some sample code that worked the same way. He downloaded it and began tinkering to see how it worked.

Then on 14 November, as he left to go to work, Spanish police met him at his door. They told him the anti-virus company Panda Software had discovered his worm had spread to 120,000 computers. When Panda analysed the worm code, it quickly discovered that the program pointed to a site Antonio had developed. Panda forwarded the information to the police, who hunted Antonio down via his internet service provider. The police stripped his house of every computer and threw Antonio in jail. After two days, they let him out, upon which his employer fired him. 'I have very little money,' he said when I met him. 'If I don't have a job in a little time, in a few months I can't pay the rent. I will have to go to

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