Be extra careful when opening documents in Windows, especially if they are Word files.
Microsoft on Friday warned that cyber criminals may be taking advantage of an unpatched flaw in the Windows operating system to install malicious software on a victim's PC.
The reported attack, now under investigation by Microsoft, involves a malicious Word document, but there may be other ways of exploiting the flaw, Microsoft said.
"Do not open or save Word files that you receive from untrusted sources or that you receive unexpectedly from trusted sources," Microsoft said in a security advisory posted to its Web site late in the day.
The flaw lies in the Jet Database Engine that is used by a number of products including Microsoft Access. Microsoft is investigating whether other programs may also be exploited in this type of attack.
Although this kind of unpatched, "zero day" attack is always cause for concern, Microsoft downplayed the risk.
"At this time, we are aware only of targeted attacks that attempt to use this vulnerability," the company said. "Current attacks require customers to take multiple steps in order to be successful; we believe the risk to be limited."
Following its usual policy, Microsoft didn't say when -- or if -- it planned to patch the bug. But in a statement sent to the press, the company did not rule out the possibility of an emergency patch, released ahead of its next set of security updates, which are expected on April 8.
Users of many versions of Word, including Word 2007, 2003, 2002, and 2000 are at risk, unless they are running Windows Vista or Windows Server 2003, Service Pack 2. Those two operating systems include a newer version of the Jet Database Engine that does not have the bug, Microsoft said.
For the technically savvy: this means that PCs with a version of the Msjet40.dll that is lower than 4.0.9505.0 are vulnerable.
There have been other reports of attacks targeting this database software recently. In December, the US-CERT (United States Computer Emergency Readiness Team) warned that attackers were sending out malicious Microsoft Access Database (.mdb) files in a similar type of attack. Security experts speculated that this exploit could have been based on a publicly reported flaw in the Jet Database Engine.