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Computerworld - Microsoft will release its emergency patch for Internet Explorer (IE) on Thursday, the company said today as it also admitted that attacks can be hidden inside rigged Office documents.

"We are planning to release the update as close to 10:00 a.m. PST as possible," Jerry Bryant, a program manager with the IE group, said in an entry on the Microsoft Security Response Center (MSRC) blog.

Yesterday, Microsoft confirmed speculation that it would issue an "out-of-band" update for the IE vulnerability, but postponed specifying a ship date until today.

Microsoft also updated the security advisory it originally published last week when it acknowledged a zero-day IE vulnerability had been used by hackers to break into the corporate networks of Google and other major Western companies. Google has alleged that the attacks were launched by Chinese attackers. Subsequently, security experts have offered evidence that links the attacks to China.

The revised advisory also addressed claims made by researchers that it's possible to exploit the newer IE7 and IE8 browsers, and even circumvent Microsoft's recommended defensive measure, DEP (data execution prevention). However, the advisory waffled on whether DEP bypass was effective, neither confirming or denying the researchers' allegations.

"There is a report of a new Data Execution Prevention (DEP) exploit," Microsoft said in the advisory. "We have analyzed the proof-of-concept exploit code and have found that Windows Vista and later versions of Windows offer more effective protections in blocking the exploit due to Address Space Layout Randomization (ASLR)."

Even a follow-up post by Jonathan Ness, an MSRC engineer, on the company's Security Research & Defense blog declined to spell out whether the DEP bypass attacks were effective. Ness, however, did reiterate Microsoft's point that the only in-the-wild attacks seen thus far have been aimed at IE6.

He also touted the additional security that ASLR and IE's Protected Mode provide, and published a table that spelled out the current attack and threat situation for IE and Windows users.

Microsoft also admitted that the vulnerability could be exploited through malicious Office documents, a vector that had not been disclosed previously. "We are also aware that the vulnerability can be exploited by including an ActiveX control in a Microsoft Access, Word, Excel, or PowerPoint file," said Bryant. "To prevent exploitation, we recommend that customers disable ActiveX Controls in Microsoft Office."

Tomorrow's update for IE will patch all attack avenues, Bryant added, including the Office document vector.

The IE vulnerability has gained considerable attention because it has been connected to the attacks that broke into Google's corporate network. McAfee was the first to reveal that the attacks against Google had been conducted using exploits of the IE vulnerability.

Since then, hackers have exploited the bug via drive-by attacks from malicious sites, and researchers have crafted new attack code that works on IE7 and IE8.

The last time Microsoft shipped an emergency security update was July 2009, when it patched IE just hours before several researchers demonstrated a critical vulnerability at a security conference. In October 2008, Microsoft rushed out a patch for Windows; that vulnerability was later exploited by the notorious Conficker worm.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, send e-mail to gkeizer@ix.netcom.com or subscribe to Gregg's RSS feed .

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