Microsoft yesterday warned of a critical vulnerability that affects users of Word running on Windows 2000 , XP and Server 2003 SP1 -- several weeks after one security company first reported an exploit and a day after a second vendor confirmed ongoing attacks.

In an advisory posted Friday, Microsoft acknowledged "public reports of very limited, targeted attacks" that exploit a bug in the Microsoft Jet Database Engine, a Windows component that provides data access to applications including Microsoft Access and Visual Basic.

According to Symantec, however, the attacks Microsoft described used malicious Word 2000, 2002, 2003 and 2007 documents, which in turn call up the vulnerable Jet .dll.

"We believe that the issue being described [by Microsoft] is one described on March 20, 2008 by Elia Florio of Symantec Security Response," the security firm told customers of its DeepSight threat analysis network on Saturday. "He notes a recent discovery, by Panda Security, of a possible zero-day exploit observed in the wild."

Panda researcher Ismael Briones had blogged about the vulnerability nearly three weeks ago, but said Microsoft had dismissed his report of an in-the-wild exploit. "Microsoft replied that they would not fix these mdb vulnerabilities, as it seems they will not acknowledge vulnerabilities which are from .mdb files," Briones said in his March 3 post.

"You appear to be reporting an issue with a file type Microsoft considers to be unsafe. Many programs, such as Internet Explorer and Outlook, automatically block these files," Briones reported Microsoft as telling him in its e-mailed response.

Last week, however, Symantec researchers analyzed an exploit that circumvented the .mdb file format blocking in Outlook by simply renaming the file to a format the e-mail client accepted. "In fact, it is possible to call msjet40.dll directly from Word, without using Access at all," claimed Symantec's Florio in a Thursday post. "In this attack, the .doc file uses mail-merge functionalities to import an external data source file and so it effectively forces Jet to load the malicious Access sample."

Florio also noted that there are at least two unpatched bugs in Jet that have been exploited by hackers. One of those bugs was reported three years ago.

Microsoft said that users running Word on machines powered by Windows Vista and Windows Server 2003 SP2 are not at risk because those operating systems include a different version of Jet.

Although Microsoft downplayed the threat, spokesman Bill Sisk confirmed that a patch was on the way. "We currently have teams working to develop an update of appropriate quality for release in our regularly scheduled bulletin process or as an out-of-band update, depending on customer impact," he said in a post to the Microsoft Security Response Center's blog.

Until a fix is available, Microsoft said users and IT administrators could disable Jet or block .mdb files at the gateway.

The next scheduled Microsoft patch day is Tuesday, April 8.