On February 24, 2009, Microsoft published Microsoft Security Advisory 968272 confirming the existence of a recently disclosed 0-day vulnerability in Microsoft Office Excel. For now, there are reports of only limited and targeted attacks attempting to exploit this vulnerability. Unfortunately, with public disclosure and exploits in limited circulation in the wild, the risk is high that more widespread attack will follow.
The flaw lies in code handling the Microsoft Office 2003 and earlier binary file formats. Microsoft confirmed that all versions of Office 2000 and later are at risk. The list of affected platforms also includes Mac OS X, with Microsoft Office 2004 for Mac and Microsoft Office 2008 for Mac being vulnerable.
Even in the absence of a security update from Microsoft, there are some good recommendations included in Microsoft's advisory.
The Microsoft Office Isolated Conversion Environment (MOICE) offers users of Microsoft Office 2003 and Microsoft Office 2007 a way to more securely open Microsoft Word, Excel and PowerPoint binary format files. KB968272 contains details on how to set MOICE as the registered handler for .XLS, .XLT, and .XLA file formats. Documents that are converted to the Office 2007 XML format with MOICE will lose their macro functionality (which depending on your perspective might not be such a bad thing). Password protected or DRM encumbered documents can?t be converted with MOICE. Mac users are unfortunately left out in the cold here, since MOICE isn?t currently supported on the Mac OS X platform.
You can also block your users from opening Office 2003 and earlier documents using Microsoft Office File Block policy. KB968272 contains details for Microsoft Office 2003 and Microsoft Office 2007 on applying registry changes to prohibit users from opening Office 2003 format documents. Office 2007 offers the ability to manage "trusted locations" that can be excluded from the File Block policy. Office 2003 users must instead use an OICEExemptions registry key if they want to exempt a directory from the File Block policy.
It remains to be seen if OpenOffice or other alternative office suites are affected by the same kind of programming flaw that caused the vulnerability in Office. Although Microsoft's new Office Open XML (OOXML) formats, and the SDLC-developed code that Microsoft wrote to implement them, do seem less at risk to these kind of vulnerabilities than the legacy formats, a move to the exchange of strictly OOXML would have its own drawbacks. Some older releases of Microsoft Office and many alternative office suites do not support the newer OOXML formats. Even users of Office 2003 must go out of their way to install additional software from Microsoft in order to open OOXML documents. When exchanging documents with partners in a business setting, the recipient's ability to easily read the attachment is an certainly an important consideration.
The Microsoft Security Response Center is a good source for updated information as Microsoft's investigation continues. I'm sure there will be new developments on this issue shortly.