The zero-day flaw, which is could let an attacker gain remote access to a person's system, affects Word 2000, Word 2002, Word 2003 and Word Viewer 2003, according to a Microsoft security advisory posted Sunday night. Word 2007 is not affected, Microsoft said.
"From the initial reports and investigation, we can confirm that the vulnerability is being exploited on a very, very limited and targeted basis," Microsoft stated in its advisory.
Nonetheless, security provider Secunia said Monday that it is rating this latest Word security flaw as "extremely critical" because it is unpatched and because malicious attackers are currently exploiting the vulnerability.
In this case, attackers are taking advantage of a flaw that arises when an unspecified error occurs when processing a Word document, Secunia said in its advisory.
Microsoft noted that the vulnerability is different from the, which also is a zero-day problem. In order to activate that flaw, a person would need to open a malicious Word file that was hosted on a Web site or an attachment that arrives via e-mail.
The software giant is not expected to havewhen it issues its monthly round of security updates Tuesday.