Microsoft's security response team has launched an investigation into reports of a zero-day attack against a previously unknown vulnerability affecting its ever-present Microsoft Word program.

The Redmond, Wash.-based software maker said it's aware of "very limited attacks" exploiting the reported Word flaw. If the vulnerability—and attack—is confirmed, the company is likely to issue a pre-patch advisory with workarounds or suggested actions or vulnerable customers.

The vulnerability was discovered during an actual live attack by anti-virus vendor Symantec. It affects multiple versions of Microsoft Word and can be used in successful code execution attacks against users of Windows 2000, Windows 95, Windows 98, Windows Me, Windows NT, Windows Server 2003, Windows XP.

According to an advisory from Symantec, the flaw is unrelated to the three previously known Word bugs that remain unpatched.

In the attack scenario discovered by Symantec, a rigged Word document arrives by e-mail with a lure to trick the target into opening the file.

"When the infected Word document is opened, it uses an exploit to drop some files onto the computer. These files are back door Trojans that enable an attacker to gain remote access to your computer," the company warned.

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Once the exploit is launched, the attacker drops a backdoor Trojan on the infected machine and immediately creates a clean Word document named "Summary on China's 2006 Defense White paper.doc."

The Trojan then checks for Internet connectivity by visiting various Web sites, such as Microsoft, Google or Yahoo and opens a back door on the compromised computer.

It then connects to the pop.newyorkerworld.com domain on TCP port 80 and uses the command prompt specified instructions to carry out basic operations, Symantec said. These could include logging keystrokes or hijacking sensitive documents and uploading them to a remote server.

"To protect yourself against these threats, do not trust unsolicited files or documents about 'interesting' topics. Do not open attachments unless they are expected and come from a known and trusted source," Symantec warned.

The latest incident closely resembles similar attacks against flaws in Microsoft Office software products, prompting speculation among security researchers that they are closely linked to corporate or even government espionage.

In December 2006, Microsoft confirmed three separate Word flaws that were being used in code-execution attacks against select targets. They remain unpatched.

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