Microsoft hopes to fix by 8 May a critical flaw in Windows Domain Name System (DNS) servers that is being exploited by online criminals.
The cmopany has been under pressure to address the flaw, reported last week, since software that exploits it has now been widely disseminated, and criminals are beginning to use it in attacks.
On Monday(17 April), security experts confirmed that variants of the Rinbot worm (also called Nirbot by some security software vendors) had been scanning networks for vulnerable systems and then attempting to exploit the DNS bug.
Microsoft characterises the level of attacks seen so far 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.
Microsoft has published a work-around for the problem, and it is the software vendor is weighing whether to step up its response and fix it ahead of 8 May, the date of the company's next scheduled security update.
"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, in an 18 April blog posting.
"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 similarly critical in the software used by Windows to read animated cursor files. Ultimately, Microsoft was forced to release an early patch for that flaw.