A new botnet is being built with the help of a worm that exploits a Windows vulnerability Microsoft fixed with an emergency patch in October, said security researchers.

Trend Micro researcher Ivan Macalintal has dubbed the worm 'Downad.a' although Microsoft gave it the moniker 'Conficker.a' and Symantec prefers 'Downadup'. Macalintal also said the worm forms part of a new botnet that criminals are creating.

"We think 500,000 is a ball-park figure," said Macalintal when asked the size of the new botnet. "That's not as large as some, such as [the] Kraken [botnet], or Storm earlier, but it's still starting to grow."

Last week, Microsoft warned that the worm was behind a spike in exploits of a bug in the Windows Server service, which is used by the operating system to connect to network file and print servers. Microsoft patched the service with an emergency fix it issued on October 23, shortly after it discovered a small number of infected PCs in Southeast Asia .

However, the new worm is a global threat, said Macalintal. "This has real potential to do damage," he said. Trend Micro has spotted infected IP addresses on the networks of Internet service providers (ISPs) in the US, China, India, the Middle East, Europe and Latin America.

Macalintal also said that it appears the botnet is being built by a new group of cyber-criminals, not one of the gangs that lost control of compromised computers when McColo, a US hosting company, was yanked off the internet. When McColo went offline, crooks lost access to the command-and-control servers which gave marching orders to some of the world's biggest botnets, including 'Srizbi' and 'Rustock'.

One result of the McColo takedown was a temporary slump in spam; some message security vendors said last week that they had seen a sharp increase in spam as the hackers managed to regain control of their botnets.

Security experts, including those at Trend Micro, are co-ordinating efforts, said Macalintal, to pass along their lists of worm-infected PCs to ISPs, who have been asked to contact the computers' owners and urge them to clean their machines of the worm.

"But that's an uphill climb," admitted Macalintal.

Users who haven't applied the emergency patch - labeled MS08-067 by Microsoft - should do so as soon as possible, Macalintal said.

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