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Computerworld - For the second time in five weeks, Microsoft Corp. warned that hackers were exploiting a critical unpatched bug in its popular Office application suite.

In a pre-patch security advisory issued late yesterday, Microsoft confirmed that attackers were using rigged PowerPoint files to trigger the vulnerability in older editions of the presentation maker. In fact, several different exploits are on the prowl, said company researchers Cristian Craioveanu and Ziv Mador in a posting to the Microsoft Malware Protection Center's blog.

Microsoft spokesman Bill Sisk downplayed the threat. "At this time, Microsoft is only aware of limited and targeted attacks that attempt to use this vulnerability," he said in an e-mail.

Unlike five weeks ago, when Sisk said the same thing about a "zero-day" flaw in Excel, Microsoft's spreadsheet software, he didn't explicitly promise that the company would patch the problem.

"Microsoft will take the appropriate action to protect our customers, which may include providing a solution through our monthly security update release process, or an out-of-cycle security update, depending on customer needs," he said Thursday. The Excel vulnerability has not yet been patched.

Yesterday's bug affects PowerPoint 2000, PowerPoint 2002 and PowerPoint 2003 on Windows, and the edition included with Office 2004 for Mac. According to Microsoft, the vulnerability is in the way that PowerPoint parses the older file format used by those versions, and can be used by attackers to run additional malware and hijack the PC.

"The question is, when will it end?" said Andrew Storms, director of security operations at nCircle Network Security Inc., referring to the regular disclosure of vulnerabilities in Office applications' file formats. "They'll probably never find all of the vulnerabilities in the file formats," he continued, "because they may not be going back into these older products to [test] them with newer fuzzers."

"Fuzzer" is the term for security development software that hammers on application inputs in an attempt to find weak spots.

"It's more likely that they're fuzzing the newer products," Storms added. "So we don't know if it's something they missed or just something they hadn't been able to find with newer fuzzers."

Other Microsoft researchers acknowledged that they had overlooked the PowerPoint vulnerability.

"The malware samples ... exploiting this vulnerability are the first reliable exploits we have seen in the wild that infect Office 2003 SP3 with the latest security updates," said Bruce Dang and Jonathan Ness, two engineers at the Microsoft Security Response Center. "Office 2003 SP3 had a good run being safe from the bad guys, but we missed this bug while back-porting fixes found in the Office [2007] fuzzing effort to Office 2003 SP3," they said in another blog posting Thursday afternoon.