Vulnerability that downed Windows 2000 computers last week also holds a serious risk for some XP users.
Computers running Windows XP with Service Pack 1 in a specific configuration are vulnerable to worm attacks similar to the ones that hit Windows 2000 systems, Microsoft said in a security advisory published Tuesday.
The Zotob worm and its offshoots, plus several other worms, downed Windows 2000 computers, including systems at ABC, CNN and The New York Times. All the worms exploited a security hole in the plug-and-play feature in Windows, for which Microsoft provided a fix earlier this month and rated as "critical" for Windows 2000.
It was previously thought that only Windows 2000 machines were vulnerable to remote attack using the plug-and-play flaw. However, Microsoft in its security advisory on Tuesday specified one scenario that also exposes select Windows XP users.
Also vulnerable are systems that run Windows XP with SP1 with file and printer sharing and the Windows guest user account enabled, according to Microsoft. This would likely be home users, because PCs are not vulnerable if connected to a network domain, which is common in business environments, Microsoft said.
"This is a minor and narrow attack scenario," said Debby Fry Wilson, a director at Microsoft's Security Response Center. "However, because Windows 2000 customers were attacked last week, we wanted to take the extra precaution of offering customers this clarifying information."
The probability that there are many vulnerable systems out there "is very remote," Fry Wilson said. Most consumers have upgraded their Windows XP machines to Service Pack 2, she said. In businesses, where Windows XP SP1 is more common, computers are not vulnerable because they are typically connected to a domain, she said.
Microsoft was made aware of the Windows XP attack possibility by security vendor Symantec, Fry Wilson said. Microsoft urges users to apply the security patches it provided earlier this month. Also, Microsoft is not aware of any attack exploiting the plug-and-play flaw that targets Windows XP.