The Web hosting company used by some of the world's biggest botnets and spammers went dark this week after several of its Internet service providers cut its connections.

As a result, the demise of San Jose, Calif.-based McColo, which has played host to botnet servers and other nefarious activity, has led to a sudden drop in worldwide spam by nearly two-thirds, according to one security firm. It has also left tens of thousands of bot machines severed from their command and control servers and unable to "phone home."

It reportedly all started with The Washington Post's contacting McColo's ISPs on Monday with information on its nefarious connections to spam and botnets gathered from various security researchers, which have long been studying McColo's connection to these activities.

The Post reported that one of the main ISPs for McColo, Hurricane Electric in Fremont, Calif., dropped McColo within an hour of investigating its activities. "We looked into it a bit, saw the size and scope of the problem you were reporting, and said, 'Holy cow!' Within the hour we had terminated all of our connections to them," Benny Ng, director of marketing for Hurricane Electric, told The Post's Brian Krebs.

McColo's connection to spamming mainly comes from its hosting several of the biggest botnets: Rustock, Srizbi, Pushdo, and Mega-D. But just because McColo is down doesn't mean the botnets are, researchers say. "McColo servers have been 'caught' hosting Rustock, Srizbi, and Pushdo botnet C&Cs. The botnets still exist despite some of the C&Cs being shut down [as a result of] the McColo shutdown," says Phillip Lin, director of marketing for FireEye.

FireEye researchers have spotted nearly 93,000 Srizbi bots trying to connect to McColo-hosted botnet command and control servers. They expect McColo to either find other ISPs to get back up and running, or for Rustock, Srizbi, and the other botnet operators to move the lost C&Cs overseas.

Symantec also says the spam respite is just short-term, and botnets will basically move more toward peer-to-peer communications. Plus, spam volumes typically jump around the holidays.

Meanwhile, aside from hosting bot, spam, and malware activity, McColo reportedly also hosted illegal content, such as child pornography. A new report from HostExploit says McColo hosted Websites, name servers, and payment services used for child pornography.

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