"This is an ongoing campaign, with new domains [hosting the malware] popping up even this morning," said Paul Ferguson, a network architect at antivirus vendor Trend Micro Inc. "The domains are changing constantly."

According to Ferguson, over half a million legitimate Websites have been hacked by today's mass-scale attack, only the latest in a string that goes back to at least January. All of the sites, he confirmed, are running "phpBB," an open-source message forum manager.

Ferguson didn't know how the sites were compromised; Trend Micro's investigation is in progress, he said. "We're not sure if it's [because of] improper configuration of phpBB or a vulnerability. Open-source applications like phpBB tend to be targeted quite a bit."

Visitors to a hacked site are redirected through a series of servers, some clearly compromised themselves, until the last in the chain is reached; that server then pings the PC for any one of several vulnerabilities, including bugs in both Microsoft's Internet Explorer and RealNetworks' RealPlayer media player. If any of the vulnerabilities is present, the PC is exploited and malware is downloaded to it.

Some of the compromised sites have been hijacked before, said Ferguson. "Some had recently been used for keyword search ranking manipulation, and others to pitch fake pharmaceuticals or just malware," he said.

Although other research by Trend Micro identified the malware hitting users' PCs as a variant of the Zlob Trojan horse, Ferguson said that more than just one piece of malware is being served. "We seeing some new stuff coming out of this one," he said.

The last massive site attack was less than three weeks ago, when sites that included government URLs in the U.K. and some domains operated by the United Nations were hacked. At the time, some researchers said that bugs in Microsoft's SQL Server or Internet Information Services server software were to blame. A few days later, however, Microsoft denied responsibility.

Don't expect the run of site infections to stop anytime soon, said Trend Micro's Ferguson. "As long as attacks are tied to site development and as long as sites don't secure their content, we'll see these attacks," he said.