Microsoft has quietly released an update to Windows XP to fix a potentially serious configuration problem in the firewall that ships as part of Windows XP Service Pack 2.

Users who installed SP2 on their Windows XP machines and also have file and printer sharing enabled may have been sharing their files and printers with the entire Internet, according to Microsoft.

By default, file and printer sharing makes changes to the SP2 firewall to give computers on the "local network" access to shared resources. However, the definition of that local network depends on the Internet service provider (ISP). In some cases, especially with dial-up ISPs, it meant the entire Internet, according to Microsoft.

"In the default configuration of Windows XP SP2, that (firewall) setting was probably a bit wider than it should have been," said Gary Schare, director of product management for Windows. "This update narrows the scope of what defines the local network."

Still, even with the update, a local network could extend beyond what users may consider a local network, Schare said. To cordon off a network and prevent unwanted access, users should place an additional firewall in front of the network, he said. For example, they could use a router with a firewall.

"If you're turning on file and printer sharing, we want you to be aware that you're sharing your files on the network, and if you are connected to the Internet, that network may be larger than you think," Schare said.

Microsoft first discussed the firewall issue in an article on its Web site in September. A "critical" update for Windows XP SP2 was released on Tuesday. However, though issued on the same day, the update was not part of Microsoft's monthly security updates. That's because security updates are only for software vulnerabilities, according to Schare.

"A vulnerability is a software bug that needs to be repaired to avoid a security issue. This is a configuration setting that shipped with Windows XP that was not optimal, but that is not classified as a security vulnerability," he said.

The update to Windows XP SP2 has been pushed out to users with the Automatic Updates feature in Windows. More information is available on Microsoft's Web site.

The IDG News Service is a Network World affiliate.