Early speculation focused on the Abobe Reader zero-day exploit as the source of the Chinese attacks on Google and other corporations earlier this week, but Adobe may be off the hook--or at least share the blame. Microsoft has determined that an unknown flaw in Internet Explorer was one of the holes used to launch the attacks which have led to Google threatening to shut down its Chinese operations.

Microsoft Security Response Center director Mike Reavey said in an e-mailed statement "This afternoon, Microsoft issued Security Advisory 979352 to help customers mitigate a Remote Code Execution (RCE) vulnerability in Internet Explorer. The company has determined that Internet Explorer was one of the vectors used in targeted and sophisticated attacks targeted against Google and other corporate networks."

Reavey continued "Microsoft continues to work with Google, other industry partners and authorities to actively investigate this issue. To date, Microsoft has not seen widespread customer impact, rather only targeted and limited attacks exploiting IE 6."

The revelation of the zero-day bug in Internet Explorer, and the release of the security advisory from Microsoft, comes just two days after Microsoft's regularly scheduled Patch Tuesday for the month of January. Coincidentally, Adobe also issued a security update this past Tuesday which addresses the zero-day flaw in Adobe Reader that has been exploited in-the-wild since at least mid-December.

Microsoft is continuing to investigate the issue, and no patch or update has yet been issued, but the security advisory from Microsoft does contain some mitigating factors you can focus on to protect your PC's from this flaw in the meantime:

• Protected Mode in IE7 on Windows Vista limits the impact of the vulnerability.

• In a Web-based attack scenario, an attacker could host a webpage that is used to exploit this vulnerability or do so via a webpage that accepts or hosts user-provided content or advertisements. In all cases, however, an attacker would have no way to force users to visit these websites and would have to convince them to do so, which is typically achieved via an e-mail or instant message.

• By default, Internet Explorer on Windows Server 2003 and Windows Server 2008 runs in a restricted mode that is known as Enhanced Security Configuration. This mode sets the security level for the Internet zone to High and so is a mitigating factor for websites that you have not added to the Internet Explorer Trusted sites zone.

• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less affected than users who operate with administrative user rights.

• By default, all supported versions of Outlook, Outlook Express, and Windows Mail open HTML e-mail messages in the Restricted sites zone, which should mitigate attacks trying to exploit this vulnerability by preventing Active Scripting and ActiveX controls from being used. However, if a user clicks a link in an e-mail message, the user could still be vulnerable to exploitation of this vulnerability through the Web-based attack scenario.

"It could also be possible to display specially crafted Web content using banner advertisements or other methods to deliver Web content to affected systems. The Microsoft investigation concluded that setting the Internet zone security setting to "high" will protect users from the vulnerability addressed in this advisory," added Reavey.

There is no indication from Microsoft yet about whether we should expect an out-of-band patch to address this issue before the next regularly-scheduled Patch Tuesday, but I wouldn't be surprised. Take a look at the security advisory and take the appropriate steps to protect your PC's from the Internet Explorer vulnerability. You should also apply the Adobe security update to protect your systems from the Adobe Reader zero-day exploit.

For more details on this issue, you can visit the Microsoft Security Research Center (MSRC) blog and The Microsoft blog. Microsoft has committed to actively monitor the situation and will post updates and additional guidance as more details are uncovered.

Tony Bradley tweets as @PCSecurityNews, and can be contacted at his Facebook page .