For the second time in nine months, Microsoft said it would not patch a vulnerability in an older product because creating a fix was "infeasible."
The omission leaves users running Office XP vulnerable to attack unless they take additional steps on their own.
Office XP, which debuted in March 2001, remains on Microsoft's list of supported suites -- users will continue to receive security updates through mid-July 2011. But on Tuesday, Microsoft said a COM (component object model) validation vulnerability in the aged suite couldn't be patched.
The decision was explained in one of the 10 updates Microsoft issued yesterday that patched a record-tying 34 vulnerabilities .
"The architecture to properly support the fixes to correct validation does not exist on Microsoft Office XP, making it infeasible to build the fixes for Microsoft Office XP products to eliminate the vulnerability," said Microsoft in the MS10-036 security bulletin. "To do so would require rearchitecting a very significant amount of the Microsoft Office XP products, not just the affected components."
Even if it managed to rework Office XP, Microsoft said the effort would "sufficiently introduce an incompatibility with other applications that there would be no assurance that these Microsoft Office products would continue to operate as designed."
"This is another example of old software showing its age," said Amol Sarwate, the manager of Qualys' vulnerabilities research lab. "The interdependencies of those [.dll files] is almost impossible to patch without upgrading the whole platform."
Instead of an actual patch, Microsoft urged Office XP users to download and run an automated tool from its "Fit it" library. The tool, said Microsoft, "provides similar protections against the vulnerability" as the patch offered to people running Office 2003 and Office 2007.
"Microsoft built a shim to protect Office XP," said Richie Lai, Qualys' director of vulnerability research. "It's a workaround, but Microsoft's not fixing the vulnerable code.
The Fix it shim can be downloaded from Microsoft's support site.
This was the second time since September 2009 that Microsoft has passed on providing a patch. Then, Microsoft declined to patch two bugs in the implementation of TCP/IP in Windows 2000 and Windows XP. TCP/IP is the Web's default suite of connection protocols. Microsoft used the same rationale last September as it did Tuesday to explain why it isn't patching.
"No, I wouldn't call this a trend," said Sarwate when asked whether the two incidents indicate a decision by Microsoft to refuse to patch older products.
Additionally, users running Office 2003 or Office 2007 must upgrade those suites before applying Tuesday's patch, Microsoft added. Office 2003 must be at Service Pack 3 (SP3), the latest major update from Microsoft, while Office 2007 must be at SP1 or SP2.
Microsoft also said that it piggybacked other changes onto the MS10-036 updates for Office 2003 and Office 2007 that address problems that resulted when a Microsoft engineer added a single extraneous "&" character to a critical code development library.
The company patched Active Template Library (ATL), a code library used by both Microsoft and third-party developers to build software, in an emergency July 2009 update .
"This update includes a defense-in-depth change ... that helps prevent components and controls built using vulnerable versions of ATL from being exploited in the Microsoft Office products," said Microsoft.
Although Microsoft didn't tell Office XP users to upgrade, Qualys' researchers did.
"Older software has the highest number of vulnerabilities," noted Lai. "Of the 14 vulnerabilities in Excel patched [Tuesday in MS10-038 ] 11 of them applied just to Office XP, but only three to the newer versions."