Attacks on legitimate Web domains, including some belonging to the United Nations, have expanded dramatically this week, security researchers said today.

One antivirus vendor said the sites might have been compromised through a "security issue" in Microsoft Web server software that has been reported to Microsoft Corp. engineers.

On Wednesday, several security companies, including San Diego-based Websense Inc., said large numbers of legitimate sites, including ones with URLs belong to the U.N., had been hacked and were serving up malware. Those latest 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.

Ryan Sherstobitoff, a corporate evangelist at Panda, said his company had told Microsoft about a problem with its Internet Information Services (IIS) Web server 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.