The software maker argues that a vulnerability recently identified is not new but a duplicate of a flaw found in December 2006.
Microsoft on Wednesday disputed a security company's claim that a fifth unpatched vulnerability in Microsoft Word was being actively exploited by criminals.
On Tuesday, Cupertino, Calif.-based Symantec notified users it had multiple exploit samples that represented new targeted attacks using a zero-day bug in Word 2003. In a warning sent to customers of its DeepSight threat management service, Symantec said that the attacks were "exploiting a previously undocumented and currently unpatched vulnerability."
Opening a malformed Word 2003 document triggers the vulnerability, which then allows the exploit -- a form of the Mdropper.x Trojan horse -- to inject several malicious files onto the PC.
Microsoft said Wednesday afternoon, however, that its research came to a different conclusion. "Microsoft's initial investigation shows that this is not a new vulnerability but a duplicate of an already known issue" first reported in mid-December, a company spokesperson said in an e-mail.
"Upon completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include issuing a security advisory or providing a security update through our monthly release process, depending on customer needs," the spokesperson added.
If Microsoft is wrong and Symantec is right, the flaw marks the fifth unpatched vulnerability in Word that has come to light since early last month. All have been used by cyber criminals in small-scale attacks against single organizations.
The most recent assault shows even more determination on the part of the attackers. "We have received different documents that use this same exploit from multiple organizations," said Eric Chien, a Symantec researcher, in a note posted Tuesday. "The documents have been each designed specifically for the targeted organization in both language and content."
The next regularly-scheduled Microsoft security updates are to debut in less than two weeks, on Feb. 13.
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