Microsoft said that attackers are exploiting a critical and unpatched vulnerability in Office 2007 using malformed documents to hijack Windows PCs and said Office 2003 and Office 2010 are also vulnerable.

The bug can be triggered by a malformed image file viewed on a website or in an email message if one of those versions of Office is installed on the system.

"We are aware of targeted attacks, largely in the Middle East and South Asia," Dustin Childs, a communications manager with the Microsoft Security Response Center (MSRC) said.

It was initially unclear exactly which versions of Windows are at risk, and thus the extent of the problem for Microsoft's customers.

While Microsoft listed only Windows Vista and Windows Server 2008 as vulnerable in its initial advisory, the McAfee security researcher who reported the flaw to Microsoft last week said that both Windows XP and Windows 7 could also be exploited through malicious Office files.

"While we spotted the attack performed via Office 2007 running on Windows XP, this is actually a fault existing in a TIFF-processing component shipped with Microsoft Office," wrote on McAfee's website. "Therefore, not only is Office 2007 with Windows XP vulnerable to this attack, but also more environments are affected, including Office 2007 running on Windows 7."

Microsoft tried to clarify the situation on its Security Research & Defense blog, but did not list every affected Windows-Office combination. According to details spelled out by MSRC engineer Elia Florio, anyone running Office 2003 or 2007, no matter what operating system powers the PC, is affected, while only those running Office 2010 on Windows XP or Server 2003 are at risk.

Office 2013, Microsoft's newest, does not contain the vulnerability, said Florio.

Microsoft later set the record straight, saying that the vulnerable scenarios are: Office 2003 and Office 2007 on all platforms; Office 2010 on XP and Server 2003 only; and all supported versions of Lync.

Childs said that Microsoft is working on a patch, but did not mention a timetable for delivering a fix.

Andrew Storms, director of DevOps at San Francisco-based CloudPassage, thought it very unlikely that Microsoft would move fast enough to put something in customers' hands next week; Microsoft's Patch Tuesday this month is slated for November 12.

"I would not expect it on Patch Tuesday," Storms said. "If it was IE, maybe. And I don't think they're taking any chances, what with the problems with some updates lately. They'll move very cautiously on this, unless their telemetry shows that attacks have really spread."

Storms was referring to several updates since April, including ones for Windows 7, the Exchange email server software and Office, that Microsoft has had to withdraw and rework after post-patching problems plagued users. Some security experts, including Storms, have wondered whether Microsoft has lost grip on its once-notable quality control.

Microsoft urged customers to apply a temporary work-around until a patch is available, and posted links to an automated "Fixit" stop-gap on a support document.