Until the flaw is patched, opening an infected document attached to an e-mail message could have serious security consequences.

"The attacker is basically loading a backdoor Trojan program onto the victim's machine and using that to steal information," explains Vincent Weafer with the antivirus company Symantec.

Hackers have already tried to exploit this flaw with government computers in Japan. Other attempts are expected, but Weafer says this current attack appears limited to corporate computers.

"In general, we don't believe this particular attack will migrate down to small businesses or home users."

Microsoft says it's working on a patch, which is scheduled to be available on June 13 as part of a new batch of security updates.

Microsoft Word Viewer does not seem to be at risk. So until the patch is out, that may be a good way to view attached documents.

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