The vulnerability has been known since Sept. 7, but until today the publicly available programs that leverage it to attack PCs haven't been able to do more than crash the operating system. A new attack, developed by Harmony Security Senior Researcher Stephen Fewer, lets the attacker run unauthorized software on the computer, in theory making it a much more serious problem. Fewer's was added to the open-source Metasploit penetration testing kit on Monday.

Two weeks ago, a small software company called Immunity developed its own attack code for the bug, but that code is available only to the company's paying subscribers. Metasploit, by contrast, can be downloaded by anyone, meaning the attack code is now much more widely available.

Metasploit developer HD Moore said Monday that the exploit works on Windows Vista Service Pack 1 and 2 as well as Windows 2008 SP1 server. It should also work on Windows 2008 Service Pack 2, he added in a .

But the code may not be completely reliable. Immunity Senior Researcher Kostya Kortchinsky said that he could get the Metasploit attack to work only on the Windows Vista operating system running within a VMware virtual machine session.

When he ran it on native Windows systems, it simply caused the machines to crash.