Storm may not be the most creative or malicious piece of malware ever written, but it's on track to become the most productive; threat researchers' recent estimates put the number of PCs it has infected at more than 1 million.

First showing up on researchers' radars about a year ago, Storm is defined by some as a worm, others as a Trojan Horse (see our Storm FAQ on page 2).

Though it has gone by many names, Storm - referring to the spam blasts it's been behind that mention storms - has stuck.

Although it doesn't use any particularly inventive or malicious techniques, such as erasing files on a hard drive or recording keystrokes to capture passwords and personal information, it has gained notoriety through its writers' ability to update and adapt both the malware's code and the spam blasts that lure people to become infected with it - all with the purpose of building a giant botnet.

"Storm is a very aggressive worm," said John Levine, co-chair of the Internet Research Task Force's Anti-Spam Research Group. "It's interesting because it uses a [peer-to-peer] control structure that makes it hard to kill."

Most threat watchers say no one knows who is behind Storm, but Finnish antivirus maker F-Secure, said a group called the Zhelatin Gang is responsible. F-Secure believes the gang is operating out of Russia. The security firm also said that Storm is the largest botnet in the world with just more than 1 million infected PCs.

Compared with highly destructive pieces of malware such as Slammer and Blaster that took down many computers and services, Storm sticks to mostly sending out spam and occasionally launching distributed denial-of-service (DoS) attacks, particularly against security companies that research the malware. But because of its size, Storm's potential for harm is serious, said Patrik Runald, technical manager at F-Secure.

"As [Storm's owners] have roughly 1 million computers under their control, we do have to take the threat of them attacking critical networks very seriously," he said.

How Storm attacks

The way Storm secretly installs itself on PCs is via spam, but typically the threat is not carried by the message. Instead the email attempts to get the recipient to visit a website that downloads the malware. It's hard to avoid Storm-related spam, which was particularly active in late summer and shows no sign of stopping. These spam blasts take advantage of whatever the malware's owners think would most entice recipients to click on the embedded link to a website purportedly related to the email's subject - be it the start of the football season or pop culture items such as computer games or a YouTube video clip.

If recipients of Storm-backed spam click on the link, they are taken to a website that automatically downloads Storm if their browsers don't have the most recent patches installed, researchers say. If visitors to the infected website do have an up-to-date browser, the site will ask them to click on a few links (with social-engineering queues such as "to download software for viewing your e-card greeting, click here") that allow Storm to circumvent current patches and install itself.

"Nobody's patched against social engineering," said Ben Greenbaum, senior manager of Symantec's Security Response team.

Once Storm is downloaded onto a PC, it turns that computer into part of its botnet, an army of compromised PCs that can be controlled remotely without the owner realising it.

And that's where the profits come in; researchers believe the people behind Storm make money by renting out portions of their botnet. Members of Storm's botnet can be turned into spam servers, so organisations looking for an untraceable way to blast spam will rent space on these compromised PCs. When the spam sender's IP address is investigated it leads back to the member of the botnet, not the actual spammer.

Considering how profitable crime on the internet has become, there's no reason to believe that Storm will die down; in August McAfee CEO David DeWalt said cybercrime has become a $105bn business, making it worth more than the worldwide illegal drug trade.

Members of the Storm botnet also can be programmed to act as web servers that download other malicious code, as well as participants in a distributed DoS attack, researchers say.