Attackers armed with an exploit toolkit have launched massive attacks in Europe from a network of at least 10,000 hacked Web sites, with infections spreading worldwide, several security companies warned Monday.

As early as last Friday, analysts reported the opening salvos of a large-scale attack based on the multi-exploit hacker kit dubbed "Mpack." The mechanics of the attacks are involved, but essentially attackers taint each compromised site with code that then redirects visitors to a server hosting the Mpack kit -- a professional, Russian-made collection of exploits that comes complete with a management console to detail which exploits are working, and against what countries' domains.

Infected computers are fed a diet of malicious code, largely keyloggers that spy out usernames and passwords for valuable accounts, such as online banking sites.

"The gang behind the attack has successfully compromised the homepages of hundreds of legitimate Italian Web sites," said Symantec researcher Elia Florio in a posting to the vendor's security response blog on Friday. "The list of compromised sites is huge and from Mpack statistics this attack is working efficiently."

Florio said Symantec is uncertain how the sites were originally hacked, but suspected a common vulnerability or configuration problem at the hosting level. Paul Ferguson, a network architect with Trend Micro Inc., would only guess at how sites were hijacked, but said that the 'how' is mostly moot. What's important: "The hackers seem to be able to find a lot of sites to compromise no matter where they look."

By Friday night, Symantec had pegged the number of compromised sites feeding Mpack exploits at 6,000; by today, Websense Inc., a San Diego-based Web security company, said it had tracked more than 10,000. "That's a phenomenal number," argued Ferguson, who said that previous compromised-site attacks using hacker kits could be counted as "several hundred here, a couple hundred there."

Screenshots of the Mpack management console posted by Websense on Monday and Symantec on Friday illustrate the large numbers of computers that have surfed to the compromised sites, and the high success rate of the Mpack-delivered exploits. Although the bulk of the victim PCs use Italian IP addresses, U.S.-based machines are not immune.

"The lion's share of the sites we're seeing are in Italy still," said Ferguson, "but we're seeing sites all over the world as well." For instance, Trend Micro has identified hacker-controlled sites hosted in California and Illinois. The California site is hosted by a company Ferguson called "notorious," but he wouldn't divulge the hosting vendor's name.

"The usual advice we give, 'avoid the bad neighborhoods of the Web,' just doesn't hold water anymore" when legitimate sites have been hacked and are serving up exploits left and right, Ferguson said. "Everywhere could be a bad neighborhood now."