After four months of gathering information from various security companies about the malicious traffic coming out of McColo Corp., a San Jose, Calif.-based Web hosting company, Krebs took his findings up the data chain and presented them to the company's ISPs, Global Crossing and Hurricane Electric.
Global Crossing declined to comment on Kreb's findings; Hurricane Electric cut off McColo's Internet service.
"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 Krebs.
According to Krebs's account in The Washington Post, McColo served spam, child pornography, rogue anti-virus sites, malware, and stolen credit card information.
Krebs said "[t]he volume of junk e-mail sent worldwide dropped drastically" on Wednesday after McColo was taken offline.
Krebs estimates that McColo helped send 75% of the spam circulating online.
"It appears that so far that the McColo shutdown has had a pretty significant effect on spam output," said Sam Masiello, VP of information security at MX Logic. He estimated that spam volume is down 50% from what it normally is.
Masiello said that getting a hosting company or ISP shut down or cut off is difficult because a lot of evidence is required. Hosting companies with a few abusive customers may also have legitimate customers.
Most legitimate hosting providers, the ones that care about maintaining their reputation, will respond to complaints and evidence of bad behavior, he said. "In instances where you're not getting the response you need, you have to escalate the problem to the next higher tier," he added.
Shutting down hosting companies may help in the short term, but Masiello observed that the people responsible for the spam and malware will probably be able to find another Web host without too much difficulty.