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\u00a0that 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\u2014an unnamed government contractor that reported details of its attack to SANS\u2014said 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\u2019 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\u2019 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.Because users must download the malicious software in order to become infected, McAfee rates the risk of the Trojan as "low." The vulnerability affects Microsoft Word XP and Word 2003, but does not work on computers that use the Word Viewer to view documents, according to Microsoft. Word Viewer is software that lets users read Word documents without installing the Word software.Microsoft is testing a fix for the Word vulnerability and expects to release this as part of its next round of monthly security patches, which\u00a0is scheduled to be released on June 13.-Robert McMillan, IDG News ServiceFor related news coverage, read Yahoo Messaging Worm Installs Bogus Browser.Check out our CIO News Alerts and Tech Informer pages for more updated news coverage.