The strange decline of computer worms

Bots, IM and email attacks in the new world of malware

Computer worms are becoming less commonplace as virus writers diversify their malware spreading tactics to create the maximum effect for the least possible effort. Email-borne worms, such as NetSky, Bagle and Sober, remain perennial favourites with malware authors but Slammer-style worms are becoming rarer, according to anti-virus firm F-Secure.

Mikko Hyppönen, director of anti-virus research at F-Secure, said that with the single exception of the Santy worm (which targeted vulnerable PHP installations) little new has been seen of computer worms since the May 2004 outbreak of Sasser. The hiatus follows a string of high-profile worm outbreaks including Blaster, Nimda, Slammer, Welchi and Code Red over recent years. The decrease in computer worms noted by F-Secure ironically comes at the same time many vendors, such as HP, Cisco, Check Point and others, are aggressively marketing worm-throttling technology.

One explanation for the dip in computer worms is that the widespread use of XP SP2 and greater use of personal firewall had rendered worms far less potent in the same way that boot sector viruses died out with Windows 95 and the introduction of Office 2000 made macro viruses far less common. Thwarted worm attacks could be explained by security improvements but Hyppönen notes that virus writers have not even attempted to spread computer worms over recent months. Computer worms exploit software vulnerabilities to spread automatically and are technically more difficult to write than other types of malware. Hyppönen reckons this factor might have a lot to do with explaining their relative decline.

Whilst standard computer worms have experienced a relative decline, email worms have remained a problem and instant message security threats are becoming a greater concern. "A worm that exploited an IM vulnerability to spread could spread very quickly and travel straight through firewalls," Hyppönen warned. Spyware, Trojans and other types of software that turn PCs into zombie drones in botnets, and the increased sophistication of viruses capable of infecting mobile phones also remain key security concerns. F-Secure recently released a free toolkit designed to root out 'stealth' software that can hide malicious programs from conventional anti-virus and anti-spyware packages, a tactic adopted in the Invisible Keylogging spyware package and viruses such as Maslan and Padodor that F-Secure expects will become increasingly common over time.

Hyppönen made his comments during the Websec 2005 Conference in London on Thursday 17 March. ®

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