Google has purged its index of the thousands of malware sites that wormed their way into results lists for hundreds of legitimate search phrases, researchers have confirmed.

"They look gone to us," said Alex Eckelberry, the CEO of Sunbelt Software, the company that broke the news earlier this week of a massive, co-ordinated campaign by attackers to spread malware through search results on Google, Yahoo, Microsoft Live Search and other sites.

"Google did confirm yesterday with us that they were working the case, and they are good about nailing this stuff," Eckelberry added in an email late Wednesday afternoon. Sunbelt had notified Google of its findings on Monday.

Earlier Wednesday, Sunbelt malware researcher Adam Thomas said his spot searches on Google the night before had come up without malware URLs. "They appeared to be zapped," Thomas said.

Ironically, Google itself refused to confirm or deny that it had cleansed its index of the more than 40,000 malware hosting sites, or even that they had existed. "Google takes the security of our users very seriously, especially when it comes to malware," a company spokeswoman said Wednesday in an email. "In our search results, we try to warn users of potentially dangerous sites when we know of them. Sites that clearly exploit browser security holes to install software, such as malware, spyware, viruses, adware and Trojan horses, are in violation of the Google quality guidelines and may be removed from Google's index."

She did not, however, answer questions about how long it takes Google to purge its search index or whether it has countermeasures that are supposed to keep sites from gaming its ranking system. According to Thomas, the group who created and stocked the sites with Trojan horses, rootkits and password-stealers drove up those sites' search-result rankings by spamming blogs and site-comment sections with their links.

Microsoft, meanwhile, would only say that it was on the case. "We are aware of the issues and are working to rectify the situation," said Justin Osmer, a senior product manager for Live Search, in an email sent by the company's public relations firm.

Yahoo did not respond to a similar request for comment.

Sunbelt first noticed the huge number of infected sites, and their appearance in results lists for a bewildering array of searches, on Sunday. Thomas explained how the attackers managed to beat the search system. "For months now, our research team has monitored a network of bots whose sole purpose is to post spam links and relevant keywords into online forms, typically comment forms and bulletin board forums," he said.

"This network, combined with thousands of pages such as the two seen above, have given the attackers very good, if not top, search-engine position for various search terms." Among the hundreds of search terms he had spotted being used were "infinity" and "hospice."

Users with PCs not completely up-to-date on their patches, he continued, were attacked by what Sunbelt has dubbed "Scam.Iwin," which turns the compromised computer into a pay-per-click zombie that generates revenue for the attacker. "Scam.Iwin is also used to load malware for other groups," claimed Thomas. "In this case, one of those malware groups is known to have been associated with the infamous RBN [Russian Business Network]."

The RBN, a notorious malware and hacker hosting organisation, made the news this month when it pulled up stakes and shifted operations from St. Petersburg, Russia, to Shanghai. Literally within days of the move to China, however, RBN abandoned the IP blocks it had been allocated there, spurring speculation by security professionals that the network had gone underground to avoid publicity, which its criminal clients naturally shun.