Exploit code has appeared for a serious Windows hole.

The flaw is in the Microsoft Distributed Transaction Coordinator (MSDTC), and was one of the holes that was patched in Microsoft Security Bulletin MS05-051 in October. According to the SANS Institute's Internet Storm Center, proof of concept code has been released that takes advantage of the flaw on vulnerable systems.

Windows 2000 systems are most directly exposed to attack, with the flaw enabling remote execution of malicious code. Other systems, such as Windows Server 2003 and Windows XP SP1, are only exposed to denial-of-service attacks, though user configuration could allows more serious breaches.

While the flaw could allow an attacker to take over a system, the exploit code, for now, only causes a system to shut down. The appearance of this exploit code, which has been published by the French Security Incident Response Team (FrSIRT), follows the private circulation of exploit code within days of MS05-051's appearance.

SANS chief technology officer Johannes Ullrich said attackers didn't seem to be using the exploit code on a large scale as of late on Sunday. "By default, port 3372 is used by the exploit," he wrote in an ISC advisory. "At this point, we see only little activity at port 3372, likely due to the fact that this PoC exploit does not actually execute any 'useful' code."

Sysadmins can apply the patch or choose one of several workarounds, as detailed in Microsoft's bulletin.

Security experts have been warning of the danger from MS05-051, and particularly the MSDTC and another vulnerability in COM+, since the bulletin's publication. These two vulnerabilities had security researchers concerned because of their similarity to the Windows Plug and Play (PnP) system vulnerability reported last August.

Within a week of its disclosure, that flaw was exploited by the authors of the Zotob worm. Variations of this attack eventually knocked hundreds of thousands of machines offline, primarily affecting Windows 2000 users.

Within hours of MS05-051's appearance, Immunity Security began distributing a working exploit for the flaw to customers of its Canvas compliance-testing service. While the exploit was kept under wraps, its very existence appeared ominous to some, who said a public exploit must be on the way soon.

Exploit code isn't the only problem caused by the bulletin so far. Some users also said that they ran into trouble when attempting to install the patch. ISC's Ullrich said the patch caused dozens of customers to report compatibility problems.