The shutdown last week of a U.S.-based Web hosting company crippled more than 500,000 bots, or compromised computers, which no longer are able to receive commands from criminals, a security researcher said Tuesday.

Although the infected PCs are still operational, the previously-planted malware that tells them what to do cannot receive instructions because of the shutdown last week of McColo.

"Half a million bots are either offline or not communicating" with their command-and-control servers, estimated Joe Stewart , director of malware research at SecureWorks Inc.

Hosting firm shutdown forces botnets to relocate

The California firm was disconnected from the Internet by its upstream service providers at the urging of researchers who believed the company's servers hosted a staggering amount of cybercriminal activity, including the command-and-control servers of some of the planet's biggest botnets. Those collections of infected PCs were responsible for as much as 75% of the spam sent worldwide; when McColo went dark, spam volumes dropped by more than 40% in a matter of hours.

The McColo takedown resulted in a record number of bots being severed from their hacker controllers by any single event, Stewart said. He compared it to last September, when Microsoft 's anti-malware utility, the Malicious Software Removal Tool (MSRT), purged nearly 300,000 infected PCs of the infamous Storm Trojan.

"That had a good impact, but it didn't stop the flow of spam globally," Stewart said of the MSRT takedown. "It didn't make a difference to other botnets that were still spamming away."

Knocking McColo offline, on the other hand, disrupted at least two major botnets -- "Rustock" and "Srizbi" -- said Stewart, and caused spam to plummet around the globe.

Stewart, a leading authority on botnets, estimated the strength of the top 11 botnets last April. Srizbi, at 315,000 bots, was No. 1 in his census, while Rustock, at 150,000, was in the No. 3 spot.

Rustock's handlers, said Stewart, may never recover control of their bots. "It does look like they're lost to them," he said, noting that those bots lack a failsafe for reconnecting with a command-and-control server if it does dark, as happened when McColo's plug was pulled.

But while Rustock's bots may be orphaned, there's a chance the Srizbi's bots can be brought back under control. "When Srizbi bots can't connect, as a backup they're coded to try other domain names," to search for new command-and-control servers, said Stewart. Those domains, however, were recently registered, perhaps pre-emptively by a security researcher who had rooted through the Srizbi code.

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