Attackers are trying to take advantage of a newly disclosed vulnerability in several of Microsoft's server products, the software giant has warned.
The flaw could allow attackers to run unauthorised code on affected computers, but attacks are limited so far, a Microsoft advisory notice said.
A spokesperson for Microsoft said the company was working on a patch but no release date has been set.
Microsoft issued seven critical patches on Tuesday 10 April, its monthly patch day, and is not due to release another round of fixes until 8 May. The company did issue an emergency patch on 3 April for a dangerous animated cursor flaw, but does not usually stray from its regular patch schedule.
The vulnerability lies in the in the domain name system (DNS) Server Service, which looks up numerical IP addresses to allow websites to be called into a browser. The affected products are Windows 2000 Server Service Pack 4, 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 could then allow code to run in the "security context of the Domain Name System Server Service", Microsoft said.
The company 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 notice.