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[ISN] Microsoft confirms Excel bug, hacks; recommends blocking files


By Gregg Keizer
January 16, 2008 

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 

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