Criminals have found a new way to attack PC users, taking advantage of what appears to be a new bug in Microsoft's Word software, according to Symantec.

Symantec warned of the attack Tuesday, saying on its Web site that it had seen attackers exploiting "what is possibly an undisclosed vulnerability affecting Microsoft Word."

The security vendor released few details of the attack, saying that it is still working with Microsoft to confirm its findings. "Initial analysis suggests that some Microsoft Office versions, even when fully patched, are affected by this exploit," Symantec said.

In a security advisory, published late Tuesday, Microsoft said that Microsoft Office Word 2002 Service Pack 3 was vulnerable to the flaw. "Our initial investigation indicates that customers who use all other supported versions of Microsoft Office Word, Microsoft Office Word Viewer, Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats, and Microsoft Office for Mac are not affected," the advisory states.

There have been "limited, targeted attacks attempting to use the reported vulnerability," said Microsoft spokesman Bill Sisk [cq] in a blog posting.

Over the past few years hackers have found a large number of bugs found in Microsoft's Office software, and this latest attack follows a familiar pattern. In order to exploit this latest flaw, attackers must trick the victim into opening a maliciously encoded Office document, which then allows them to install malicious software on the PC, Microsoft said.

In this case, the malicious code is a Trojan horse program, called Backdoor.Darkmoon, which logs the victim's keystrokes in order to steal passwords.

Symantec's antivirus software is now detecting the attack, but the security company recommends that users avoid opening unsolicited Word documents.

This is the second Microsoft attack reported this week. On Monday, Microsoft said that cybercriminals are exploiting a bug in software used by its Access database program. That flaw lies in the Snapshot Viewer ActiveX control, which ships with "all supported versions of Microsoft Office Access except Microsoft Access 2007," Microsoft said in a security advisory.