Attacks on legitimate Web domains, including some belonging to the United Nations that began earlier this week, have expanded dramatically, security researchers said Friday, with hundreds of thousands of pages hacked by Friday.

One anti-virus vendor said the sites might have been compromised through a "security issue" in Microsoft's Web server software that has been reported to Microsoft's engineers.

Last week, several security companies, including California-based Websense, said large numbers of legitimate sites, including URLs for the U.N., had been hacked and were serving up malware. These latest site compromises were only the most recent SQL injection attacks, however; similar attacks have been launched since the first of the year, and were last detected in large numbers in March.

Earlier in the week, Dan Hubbard, Websense's vice president of security research, estimated the number of hacked sites in the low six figures. By today, that number had soared as firms such as Panda Security pegged the number at 282,000, and F-Secure said its infected-page count was above half a million.

Ryan Sherstobitoff, a corporate evangelist for Panda, said his company had reported a problem with Internet Information Services (ISS) to Microsoft that was probably responsible for the hacks. "We reported a security issue, but I don't have any specific details on whether it's a vulnerability," Sherstobitoff said.

"It's not like this is a brand-new problem," he said, referring to legitimate site compromises. "But Microsoft has already issued a security advisory that said they are investigating public reports of problems with IIS. This seems to be related to that advisory."

That advisory was published April 17, and warned users of a bug in most versions of Windows that could be exploited through custom Web applications running in IIS. It could also be exploited via SQL Server, Microsoft said.

On Friday, Microsoft said it did not know whether the ongoing site attacks were linked to the bug described in the April 17 advisory. "We have not yet determined whether or not these reports are related to Microsoft Security Advisory 951306 released last week," a company spokesman said in an e-mail.