Hackers are exploiting a vulnerability in Microsoft Word to plant malware on Windows PCs, Microsoft said Tuesday.
The bug in Microsoft Word 2002, 2003, 2007 and 2010 was patched Nov. 9 as part of Microsoft's monthly security update.
Word 2008 and 2011 for the Mac have also been patched, but Microsoft has not yet issued a fix for the same flaw in the older Word 2004. The circulating attacks affect only Windows versions of the suite, however.
According to the Microsoft Malware Protection Center (MMPC), the group that investigates attack code and issues signature updates for the company's antivirus software, the first in-the-wild exploits were detected last week.
When Microsoft shipped the Word patch last month, it rated the bug as "1" on its exploitability index, meaning it believed a working attack would pop up within 30 days.
The attack uses a malicious RTF (Rich Text Format) file to generate a stack overflow in Word on Windows, said MMPC researcher Rodel Finones. Following a successful exploit, the attack code downloads and runs a Trojan horse on the compromised computer.
Finones said that the code "reliably exploits this [Word] vulnerability."
Last month, Microsoft rated the RTF vulnerability as "critical" in Word 2007 and 2010, but as "important" in all other affected versions.
At the time, outside researchers had put their bets on the bug as a hacker choice because users running Office 2007 or 2010 could be attacked if all they did was preview a specially-crafted RTF document in the Outlook e-mail client.
"Once a [malformed] message hits the Outlook preview pane, remote code can be executed. You should patch this right away," Jason Miller, the data and security team manager for Shavlik Technologies, said on the day Microsoft released the patch.
Finones urged users who have not yet installed the November patch to do so as soon as possible.
More information about the vulnerability can be found in the MS10-087 security bulletin.
The MS10-087 update can be downloaded and installed using Microsoft Update and Windows Server Update Services (WSUS).
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is firstname.lastname@example.org.