Exploit code is circulating on the internet that can take advantage of a critical Windows security vulnerability disclosed by Microsoft last month.
Microsoft issued a patch against the security hole but the rogue code aims to crash the machines of users who have not deployed the fix.
The vulnerability is related to a bug in a Windows component for transaction processing, called the Microsoft Distributed Transaction Coordinator.
Microsoft addressed the flaw in security bulletin MS05-051.
The malicious code circulating is designed to launch a denial-of-service attack on networks, leading to them ultimately crashing under the weight of data being created from such an attack.
The company says it is not aware of any attacks on Windows-based PCs using the exploit code.
Microsoft is set to issue its latest batch of security updates as part of its monthly release cycle on 13 December.