A combination of the long Christmas holiday and some clever, sexy Christmas-themed spam emails helped criminals behind the Storm worm more than double their network of infected PCs, security experts say.

On the day before Christmas Storm sent off a flurry of email that invited victims to visit a Christmas-themed strip show on websites such as Merrychristmasdude.com. Victims who downloaded the strip show found their PCs attacked by malicious software.

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This site, and about 14 other Storm-related domains, was registered using a Russian domain name registrar called Nic.ru, where staff was largely unavailable during the holidays, according to Richard Cox, the chief information officer with the Spamhaus anti-spam effort.

"The trouble was they were quite simply out to Christmas lunch," Cox said. "And they didn't get back until Wednesday."

Spamhaus representatives tried to contact the registrar on December 26, but they soon discovered that the company was in essence shut down. Four days later they received an email from a Nic.ru employee saying that they would have to wait until staff returned to work in January before anything could be done, Cox said.

Storm's creators took advantage of another common problem in the domain name registration system. They targeted a registrar that did not have an established policy for taking down malicious domains, so that gave the criminals a little more time to run their scam, Cox said.

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By Wednesday of this week, Nic.ru had removed the Storm-related domains from its database, knocking the criminal network's websites offline.

But now security experts say that Storm has more than doubled in size, adding about 25,000 PCs to a network that had been 20,000 strong.

Storm's creators also changed the configuration of their malware, making it harder for security software to detect it, and this helped inflate the number of infections, Cox said.

The Christmas campaign was "pretty effective at growing the botnet", said Jose Nazario, senior security engineer at Arbor Networks. "The contributing factors there were clearly the successful timing, tied to a major holiday, coupled with tweaks to the malware to avoid antivirus detection."

Storm Worm has been attacking computer users since January 2007, when it began tricking victims into downloading malicious software, claiming that it was a video of violent storms that had been ravaging Europe.

Since then it has been one of the most virulent sources of malware, although it has shrunk in size as detection methods have improved. Recently the network appears to have begun renting out its infected PCs to phishers, according to some researchers.

"Storm is an insidious pest. We've learned how to manage it," said Nazario. But, he added, the success of this Christmas campaign proves that the network can still be a serious threat. "They're great marketers," he said.