Cisco is advising its Cisco Security Agent users that a bug in the software might cause problems in Windows. The company said that users should upgrade the product .

The flaw lies in a driver used by the client software. By sending maliciously crafted data to the PC, attackers could create a buffer overflow condition in the Windows kernel, causing the system to crash.

Security firm Secunia rates the crash flaw "moderately critical," but the vulnerability could be used by attackers to also run unauthorised software on the Windows machine, Cisco warned.

Cisco released hotfixes for the bug on Wednesday. A large number of Cisco products install this agent, including the Cisco Security Manager, Cisco Unified Communications Manager and the Cisco Voice Portal.

If attackers find a way to exploit this flaw to run code, it could become a serious security problem.

"Back in 2004, such a vulnerability would probably have led to a flurry of noisy network worms," wrote Daniel Wesemann, a handler with the SANS Internet Storm Center, in a blog posting. "Today, drive-by installs of spyware are more likely, but at least as damaging. The bottom line is still the same: If you are using the vulnerable component, patch as soon as possible."

The buffer overflow can be caused by sending a malicious TCP (Transmission Control Protocol) packet to ports 139 or 445, the ports used by the Microsoft Server Message Block file-sharing protocol.