Microsoft confirms Excel bug, hacks; recommends blocking files

Vendor suggests users cut access to earlier formats, exercise MOICE

Ongoing attacks are exploiting a flaw in most versions of the popular Excel spreadsheet application, Microsoft Corp.'s security group said late Tuesday.

The attacks, which the Microsoft Security Response Center (MSRC) downplayed as "targeted, and not widespread," are using a bug found in Excel 2000, Excel 2002, Excel 2003 Service Pack 2, Excel Viewer 2003 and Excel 2004 for Mac. Newer editions -- Excel 2003 SP3, Excel 2007 and Excel 2008 for Mac -- are not vulnerable, Microsoft claimed. That last version, Excel 2008 for Mac, launched earlier Tuesday at the Macworld Conference & Expo in San Francisco.

"Microsoft is aware of specific targeted attacks that attempt to use this vulnerability," said Tim Rains, the security response communications lead at Microsoft, in an e-mail forwarded by the company's public relation firm. "Microsoft is aggressively investigating the public reports and customer impact."

According to the security advisory Microsoft posted Tuesday night, the vulnerability -- which it did not specify -- could let attackers jimmy a PC sufficiently to snatch control from the rightful owner.

The likely attack vectors, said the advisory, would be to attach a malformed document to e-mail or stick it on a Web site, then convince users to open the file.

Office file format vulnerabilities, even vulnerabilities specifically within Excel, are not new. Attackers have uncovered -- and used -- a wide array of bugs in Word, Excel and PowerPoint documents for nearly two years, often in pinpoint attacks that seed a very small number of businesspeople with spam that includes attached files.

In lieu of a patch -- which Microsoft did not promise it would produce -- the company recommended that Office 2003 users run suspect Excel files through MOICE (Microsoft Office Isolated Conversion Environment), a free conversion tool released last year that converts Office 2003 format documents into the more secure Office 2007 formats to strip out possible exploit code. Alternately, it told administrators they could block all Office 2003 and earlier formats except those in "trusted locations" by using File Block, a last-ditch defense that requires editing the Windows registry or modifying Group Policy settings.

Ironically, file blocking -- albeit enabled by default first in Office 2007, then in September's Office 2003 SP3 update -- has raised a minor ruckus in the past week as users complained of the practice, and Microsoft tried to calm the waters by making it slightly easier to unblock the older, but banned, formats.

The last time that Microsoft patched any edition of Excel was in August 2007, when it issued MS07-044, an update that fixed a similar document format flaw in Excel 2000, Excel 2002, Excel 2003 and Excel 2004 for Mac.

Copyright © 2008 IDG Communications, Inc.

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