Microsoft is looking to fix a critical flaw in Windows Domain Name System (DNS) servers by 8 May, the date of its next scheduled security update. The company is currently weighing up whether to address the problem earlier.

Microsoft has been under pressure to address the flaw since software that exploits it has now been widely disseminated, and criminals are beginning to use it in attacks.

On Monday, security experts confirmed that variants of the Rinbot worm (also called Nirbot by some vendors) had been scanning networks for vulnerable systems and then attempting to exploit the DNS bug.

Microsoft characterises the level of attacks as "not widespread."

The problem affects Windows 2000 and Windows Server 2003 systems, which can be tricked into running unauthorised software when an attacker sends them maliciously encoded RPC (Remote Procedure Call) packets to the DNS server. The latest versions of Windows 2000 Professional, XP and Vista are not vulnerable to this attack.

"While we don't have a firm estimate on when we'll complete our development and testing of updates for this issue, we have teams around the world working on it twenty-four hours a day, and hope to have updates no later than May 8, 2007," wrote Microsoft security program manager Christopher Budd on the Microsoft Security Response blog. "However, this is a developing situation and we are constantly evaluating the situation and the status of our development and testing of updates," he added.

Microsoft was in a similar situation three weeks ago when attackers began exploiting a critical Windows flaw to read animated cursor files. Ultimately, Microsoft was forced to release an early patch for that flaw.

Original story by IDG News Service