Computer worms are set to become a more deadly combination of virus writing and hacker exploits, according to security experts at Symantec. Code Red and Nimda marked the demise of socially engineered worms, by combining a blended threat of proven hacker exploits. Both worms attacked the same buffer-overflow vulnerability in Microsoft's IIS software, while Nimda additionally incorporated a mass-mailing component enabling the virus to propagate on a massive scale. Neither of the worms relied on the traditional need for an infected computer user to double-click on a malicious attachment. "Nimda and Code Red have eliminated the need for human intervention, by virus writers using what hackers have already provided," said Eric Chien, chief researcher at Symantec. "One year ago email worms were the big threat, as they spread quickly and far -- but now a lot more virus writers will be looking at the hacker worm." Chien predicts that by next year, the "blended" threat of computer worms could be enough to cause a serious Internet slowdown. Antivirus experts at Symantec have already developed an algorithm to prove that by removing human interaction from the virus equation, every PC connected to the Internet could be affected by a single worm within 20 minutes. But the trend towards blended virus attacks is blurring the lines of responsibility for computer worms. On Wednesday, Microsoft launched a verbal attack on security firms and hackers who release what it calls virus "blueprints". A study done by Microsoft on recent attacks by worms such as Code Red and Nimda found that each had been prefaced by the release of so-called exploit code -- sample programs created by security firms and hackers to exploit software flaws. "Responsibility lies with the people who release the worm, not necessarily the people who wrote it," said Chein. The Anna Kournikova virus, for example, was written with the help of an existing virus toolkit available on the Internet, but Chein argues that the script kiddie who unleashed the virus is the person ultimately responsible for any damage caused to the networks. The changing trend in computer viruses is also likely to affect the structure of IT security companies. Hacker worms will make it necessary for antivirus units to merge with intrusion detection systems, according to Chein. "Companies who only concentrate on the antivirus side won't survive," he concluded. See the Viruses and Hacking News Section for the latest headlines. See the Net Crime News Section for the latest on hacking, fraud, viruses and related issues. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Security forum. Let the editors know what you think in the Mailroom. And read other letters.