The attacks are limited so far, according to a Microsoft advisory issued late Thursday evening. The company is working on a patch, but no release date has been set, a spokeswoman in London said.

Microsoft just issued seven critical patches on Tuesday, its monthly patch day, and isn't due for another round until May 8. The company did issue an emergency patch on April 3 for a dangerous animated cursor flaw, but it does not typically stray from its regular patch schedule.

The vulnerability lies in the in the DNS (domain name system) Server Service, which looks up numerical IP addresses to allow a website to be called into a browser. The affected products are Windows 2000 Server Service Pack 4, and Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2.

The flaw can cause a stack-based buffer overrun in the DNS Server's remote procedure call (RPC) interface. RPC is a protocol through which a program can request a service from an application on another machine on a network.

An attacker could try to exploit the problem by sending a special RPC packet to the system, which then could allow code to run in the "security context of the Domain Name System Server Service," Microsoft said.

Microsoft listed several ways to block an attack until a patch is issued. Administrators can disable RPC's remote management capability through the registry key settings, Microsoft said. The SANS Institute, which monitors the health of the Internet and conducts security training, recommended this option.

Other workarounds include blocking ports 1024 to 5000 on the firewall, which are used by the RPC protocol, and enabling advanced TCP/IP filtering, Microsoft said.

Danish security vendor Secunia rated the problem as "highly critical" in an advisory.