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Microsoft patches critical Windows 7 bug, downplays exploit threat

No fix for Duqu flaw, but quashes different bug in same TrueType parsing engine

Microsoft today delivered four security updates that patched four vulnerabilities in Windows, most of them affecting the newer editions of Vista and Windows 7.

Only one of the updates was marked "critical," Microsoft's most-serious threat ranking. Two of the remaining were labeled "important" and the fourth was tagged as "moderate."

As expected, Microsoft did not patch the Windows kernel vulnerability exploited by the Duqu campaign.

Top on Microsoft's chart today -- and on outside researchers' to-do lists as well -- was the MS10-083 update that patches a bug in Windows Vista's, Windows 7's and Server 2008's TCP/IP stack, which regulates Internet connections.

The vulnerability could be used by attackers in certain circumstances to hijack an unpatched PC, said Microsoft, which nevertheless downplayed the likelihood of successful attacks.

"This critical bug allows an attack via the network, and looks troublesome at first glance," said Andrew Storms, director of security operations at nCircle Security. "But it doesn't look very easy to pull off, so in this case, it's not as big a concern as one would think."

Storms pointed to a post by Microsoft engineers on the Security Research & Defense blog that spelled out the necessary conditions for an effective attack.

"We believe it is difficult to achieve [remote code execution] using this vulnerability considering that the type of network packets required are normally filtered at the perimeter and the small timing window ... and [that] a large number of packets are required to pull off the attack," wrote Ali Rahbar and Mark Wodrich of the Microsoft Security Response Center (MSRC).

Microsoft gave the vulnerability an exploitability index rating of "2," meaning that it expects only unreliable exploit code to appear in the next 30 days.

Even so, some researchers warned that if criminals focused their attention on the bug, they may be able to craft a consistent exploit that could be used to launch worm-based attacks.

Microsoft also updated Windows Mail and Windows Meeting Space on Vista, Windows 7 and Server 2008 to fix yet another "DLL load hijacking" vulnerability.

DLL load hijacking, sometimes called "binary pre-loading," describes a class of bugs first revealed in August 2010. Microsoft has been patching its software to fix the problem -- which can be exploited by tricking an application into loading a malicious file with the same name as a required dynamic link library, or DLL -- since last November.

Today's MS11-085 update was the eighteenth Microsoft has issued to fix DLL load-hijacking vulnerabilities in its software.

"They're a dime-a-dozen these days," said Storms of the latest in the long-running series.

Researchers also noted that while Microsoft did not patch the Duqu-exploited bug, it did fix a different flaw in the TrueType font parsing engine , the component targeted by the Trojan's attacks.

MS11-084 fixes a single vulnerability in the Windows kernel-mode driver "Win32k.sys" that can be exploited through a malformed TrueType font file.

"We're see a pattern of kernel-level bugs and parsing of font files," said Storms. "And they're going to have to come back and patch this again for Duqu."

Microsoft patched the TrueType engine within Win32k.sys just last month, fixing a flaw that let hackers conduct denial-of-service attacks to cripple Windows PCs. Today's bug was also categorized as a denial-of-service flaw.

In lieu of a fix, Microsoft last week told customers that they could defend their systems by blocking access to "t2embed.dll," the dynamic link library that handles embedded TrueType fonts.

An advisory offered command-prompt strings IT administrators can use to deny access to t2embed.dll, and links to one of Microsoft's "Fix-it" tools that automate the process of blocking or unblocking access to the library.

Blocking t2embed.dll, however, has side effects: Applications, including Web browsers, applications in Microsoft's Office suite and Adobe's Reader, may not render text properly.

Microsoft also updated that advisory today with a link to a list of its antivirus partners that have issued signatures to detect the kernel-based Duqu attacks.

November's security patches can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer , on Google+ or subscribe to Gregg's RSS feed . His e-mail address is [email protected] .

See more articles by Gregg Keizer .

Read more about security in Computerworld's Security Topic Center.


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