Microsoft has acknowledged that exploit code is circulating for a vulnerability it acknowledged six months ago, but has yet to patch.

It's not clear whether Microsoft intends to fix the flaw this week.

On Thursday, Microsoft revised a security advisory it first posted April 19 about a bug in Windows XP, Vista, Server 2003 and Server 2008 that could be exploited to gain additional privileges on vulnerable machines. "Exploit code has been published on the Internet for the vulnerability addressed by this advisory," confirmed Bill Sisk, a communications manager at Microsoft's Security Response Center in a post to the MSRC blog.

The vulnerability has a convoluted history.

In late March, Argentinean security researcher Cesar Cerrudo announced he had found a bug that could let attackers bypass some of the security schemes in the newest versions of the operating system, including Windows Server 2008. At the time, Sick called Cerrudo's bug a "design flaw" rather than a vulnerability, and downplayed the threat.

Only after Cerrudo presented his findings at a security conference in April in the United Arab Emirates did Microsoft change its tune and call the flaw a security problem.

On Wednesday, Cerrudo posted a proof-of-concept (PoC) exploit for the months-old vulnerability. "It has been a long time since [my April] presentation was published so I decided to release a PoC exploit for Windows Server 2003 that allows [you] to execute code under [a] SYSTEM account," Cerrudo said in the description of his exploit on milw0rm.com.

"Basically, if you can run code under any service in Windows Server 2003 then you can own Windows," he added. "So if you can run code from an ASP .NET or classic ASP web application, then you can own Windows, too."

Microsoft has yet to issue a fix for the flaw; since April its own move has been to recommend workarounds for customers running Internet Information Services (IIS).

Not only has Microsoft not patched the problem, but it continued to be noncommittal about a fix yesterday. "Microsoft will take the appropriate action to protect our customers, which may include providing a solution through a service pack, our monthly security update release process, or an out-of-cycle security update, depending on customer needs," the advisory's boilerplate language read.

Sisk didn't promise a fix in his blog post, either. "We will continue to monitor the situation and post updates to the Advisory and the MSRC Blog as we become aware of any important new information," he said Thursday.

Also on Thursday, Microsoft published its monthly pre-patch notice outlining what would be fixed next week. Although six of the 11 expected updates will affect Windows, and two of those six will affect the editions called out by the April advisory, Microsoft does not provide enough detail prior to patching to determine whether one of those will fix the privilege elevation flaw.

Both of the potential updates were labeled as "important," Microsoft's second-from-the-top threat rating; the company typically ranks privilege elevation vulnerabilities as "important."

Microsoft will release October's security updates at approximately 1 p.m. EST on Tuesday.