The seemingly endless spate of worm infestations over the last year has left something even more troubling in its wake: armies of zombie PCs that can be used to send spam, attack Web sites, and generally wreak havoc over the Internet.

Worms such as Sobig, MyDoom, and Bagle have been identified as containing malicious code (malware) that allows remote attackers to take over infected machines -- while their victims are blithely oblivious.

Spreading Nasties

UK security firm Sophos estimates that 40 percent of spam is now sent by zombie machines. Sandvine, a network security firm, puts the figure at 80 percent. Distributed computing company Akamai Technologies Inc. blames zombie PCs for a denial of service attack that briefly blacked out sites like Google, Microsoft and Yahoo in June. Reuters reports that British teen hackers are hiring out their zombie networks for around US$100 an hour.

Besides relaying spam and launching DOS attacks, a zombie machine can be used to send phisher scams, spread viruses, download pornography, and steal personal information, says Carole Theriault, a Sophos security consultant.

"Basically, it is a complete invasion of privacy that can leave you penniless, can have your computer send out all kinds of nasties to innocent computers, and as part of the collective--sorry for Star Trek terminology--contribute to the cyberhavoc going around," Theriault says.

Are You a Zombie?

Sophos estimates half a million zombie PCs are operating worldwide; other sources put the figure as high as two million. A recent Earthlink study hinted of widespread malware installations. Those numbers are likely to climb even further, says Steve Gibson, president of Gibson Research Corporation and well-known PC security guru.

"There's a tremendous incentive for hackers to infect other people's PCs," Gibson says. "They don't care about your financial records, letters to your mother, or pictures of your family album. All that machine represents is bandwidth they can use for targeting other people."

Determining whether your PC is a zombie isn't always easy, says Fred Felman, vice president of marketing for Zone Labs Inc., a San Francisco security software maker. Symptoms can include a suddenly sluggish broadband connection, excessive hard drive activity, an unresponsive mouse or keyboard, or bounce notifications in your inbox from people you never tried to contact. Yet you could show all of these symptoms and still not be infected.

Experts agree that you can reduce your risk by installing a personal firewall and antivirus software, and keeping your Windows Updates up to date. Yet most home users remain woefully unprotected. A study conducted in May 2003 by the National Cyber Security Alliance found that two-thirds of home users did not have a properly configured firewall.

Later this summer Microsoft plans to release XP Service Pack 2, which will feature a beefed-up firewall and other security enhancements designed to reduce remote access to PCs. But Gibson fears widespread adoption of SP2 will cause new problems by creating a single point of attack for malware to defeat.

Good Fences, Good Neighbors

Even security-savvy users are at risk. Zone Labs' Felman says his own notebook was infected by the Sasser worm while he was attempting to uninstall one firewall and install another. He says users need to take a neighborhood-watch approach to fighting malware.

"We're all responsible for looking out for weird behavior in airports and our neighborhoods; we should also be looking out for weird behavior on the network," he says. "And we need to start by looking at our own machines."