"Our investigation has shown that there are no new or unknown vulnerabilities being exploited. This wave is not a result of a vulnerability in Internet Information Services or Microsoft SQL Server," said Bill Sisk, a communications manager at Microsoft's Security Response Center, in a note posted to the group's blog.

Sisk said the post was in response to reports that over half a million pages, including some belonging to the United Nations, have been compromised by SQL injection attacks. Once hacked, those sites were modified to download malware to visitors' PCs.

Early on Friday, Panda Security said it had notified Microsoft of what it called a "security issue" in the company's Web server, Internet Information Services (IIS). However, Panda stopped short of dubbing the problem a "vulnerability."

Sisk said that the site hacks were essentially run-of-the-mill SQL injection attacks. "[They] are not issues related to IIS 6.0, ASP, ASP.Net or Microsoft SQL technologies," he claimed.

Microsoft's IIS team also chimed in to deny that the attacks were exploiting any bugs, known or not, in its software. "For end users, the investigation also shows no indication of an unpatched vulnerability in IIS, SQL Server, Internet Explorer or any other Microsoft client software, so we recommend customers apply the latest updates to be protected from these attacks," said Bill Staples, an IIS product manager, in an update posted to the software's support forum on Friday night.

And although there has been speculation that the attacks were related to a vulnerability mentioned in an April 17 advisory, Sisk said that wasn't true either. "We have also determined that these attacks are in no way related to Microsoft Security Advisory 951306," he said.

Sisk and Staples urged Web site developers to follow Microsoft's guidelines to protect their domains from SQL injection attacks.