Microsoft's Web site became largely inaccessible at about 12:50 p.m. PST, according to reader reports received by CNET News.com. The site appeared to be back up and running around 2:15 p.m.
The outage occurred as system administrators and security experts were bracing for a potentially large Internet attack. The U.S. federal government warned earlier this week that an attack could be brewing that exploits a widespread flaw in Microsoft's Windows operating system.
Microsoft spokesman Sean Sundwall said Microsoft was the victim of a denial-of-service attack, but he stressed that no Windows vulnerability was exploited.
"We're investigating the cause of the issue," Sundwall said. "One thing we do know is it is not an issue associated with any vulnerabilities. It was a very traditional denial-of-service" attack.
The software maker announced a patch for the flaw two weeks ago. However, the outage temporarily prevented some customers from reaching Microsoft's security patches.
The U.S. Department of Homeland Security issued an updated advisory this week about possible hacker attacks on computers that run Microsoft operating systems. The advisory warned that several exploits are now in widespread distribution on the Internet.
"These exploits provide full remote system level access to vulnerable computers," the advisory stated.
No worm code had been reported by Friday afternoon. But the Homeland Security Department said there is evidence to show an increase in searches for vulnerable computers on the Internet over the past week. This reinforces the urgency to install patches on computers that use Windows operating systems as soon as possible, the advisory said.
Sundwall said Microsoft is working with law enforcement to try and track down those responsible for the attack, which comes amid the annual DefCon conference in Las Vegas.
