Microsoft Word users should be extra careful about the files they download because hackers are exploiting an unpatched vulnerability in the popular word-processing software.

On Thursday, security vendor McAfee warned users of a new Trojan program, called BackDoor-CKB!cfaae1e6, that secretly installs software on a computer. For the Trojan to work, however, hackers must first trick users into opening a malicious Word document. Once that has been done, though, the results can be nasty.

Unlike viruses and worms,Trojan programs do not make copies of themselves that keep spreading throughout the Internet. Hackers directly distribute the programs, which are often disguised as useful or interesting downloads.

Once installed, the malware lets hackers "execute any external commands, download additional Trojans, capture desktop screen shots, monitor and record keystrokes or passwords," McAfee said in a statement on its website.

Symantec has confirmed that hackers are circulating the malware via malicious Word document e-mail attachments. But at present, its use is "limited to attacks against select targets," Symantec said in a note on its DeepSight threat analysis service.

The attack originated in Asia and targets "specific large organizations," Symantec said, adding that it has seen similar targeted attacks in the past that also took advantage of flaws in Microsoft Office applications.

The attackers behind the Trojan may be operating from China or Taiwan, according to Johannes Ullrich, chief technical officer of the SANS Internet Storm Center. Servers associated with the attack have been traced back to those countries, and researchers have found Chinese characters in the malicious Word document, Ullrich said.

One company—an unnamed government contractor that reported details of its attack to SANS—said the malicious e-mail had been sent to only one person in its organization, and had been written to resemble a normal inter-office message, Ullrich said.

"The exploit was quite sophisticated," he said. "None of the antivirus systems that they used caught it."

Ullrich said he did not know what the attackers’ ultimate goal may be, but they can snoop on data or install unauthorized software once the Trojan gets installed. "It opened up a remote connection to a website in China that would have allowed it to remotely control [the infected computer]," he said.

SANS has published a number of tips on how to avoid this type of attack. The security training organization recommends that companies limit users’ privileges and monitor outbound traffic. It also suggests that companies think about quarantining all attachments for six to 12 hours in order to give the antivirus vendors time to catch up with new threats.