Microsoft today patched 15 vulnerabilities in Windows, Windows Server, Excel and Word, including one that will probably be exploited quickly by hackers. None affect Windows 7, the company's newest operating system.

The 15 flaws fixed in Tuesday's six security updates were less than half the record 34 Microsoft patched last month in 13 separate bulletins. Of today's 15 bugs, three were tagged "critical" by Microsoft, while the remaining 12 were labeled as "important," the next-lowest rating in the company's four-step severity scoring system.

Experts agreed that users should focus on MS09-065 first and foremost. That update, which was ranked critical, affects all still-supported editions of Windows with the exception of Windows 7 and its server sibling, Windows Server 2008 R2.

"The Windows kernel vulnerability is going to take the cake," said Andrew Storms, director of security operations at nCircle Network Security. "The attack vector can be driven through Internet Explorer, and this is one of those instances where the user won't be notified or prompted. This is absolutely a drive-by attack scenario."

Richie Lai, the director of vulnerability research at security company Qualys, agreed. "Anyone running IE [Internet Explorer] is at risk here, even though the flaw is not in the browser, but in the Win32k kernel mode driver."

Both Storms and Lai were referring to the one bug marked critical in MS09-065, which actually patched a trio of vulnerabilities. According to Microsoft, the Windows kernel improperly parses Embedded OpenType (EOT) fonts, which are a compact form of fonts designed for use on Web pages. EOT fonts, however, can also be used in Word and PowerPoint documents.

Hackers could also launch attacks by attaching Word or PowerPoint documents to e-mail messages, then duping users into opening those documents.

In lieu of patching the problem, users can easily block the most likely attacks by disabling IE's support for embedded fonts. "That's a low-impact mitigation," Lai said. "The worst that could happen is that some sites might look ugly." His advice would still leave PCs open to attack via malicious Word or PowerPoint documents, a point Microsoft also made in the vulnerability's write-up.

Because Windows 7 and Windows Server 2008 R2 were not affected by the MS09-065 update, Storms and Lai assumed that Microsoft caught the bug before it wrapped up the final code, or release to manufacturing (RTM) build, of the operating system, and is only now getting around to plugging the holes in Windows 2000, XP and Vista, as well as Server 2003 and Server 2008.

"Windows 7 Release Candidate [RC] is probably vulnerable," said Storms, citing Microsoft's policy of not providing security updates for preview versions of an operating system when the final has been released. "That's why you don't see Microsoft patching Windows 7 RC or Beta," said Storms. "For anyone still running RC, they should take heed and upgrade to the RTM."

But while Storms speculated that Microsoft knew the EOT font flaw was a security issue -- and waited until now to patch older Windows -- Lai thought that Microsoft didn't realize until recently that it was also a security vulnerability in editions prior to Windows 7. "I think they fixed this bug as part of the code sanitization during [Windows 7's] development cycle. It was actually only publicly disclosed recently, and then they patched it in other Windows."

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