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 (Compare Patch and Vulnerability Management products).

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.”

There have been a large number of bugs found in Microsoft’s Office software, including Word, over the past few years. In order to exploit these flaws, attackers must typically trick the victim into opening a maliciously encoded Office document, which then allows them to install malicious software on the PC.

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.