Duqu exploits same Windows font engine patched last month, Microsoft confirms

Offers automated tool to block access to TrueType embedded fonts while it works on patch

Microsoft on Thursday confirmed that the Windows kernel vulnerability exploited by the Duqu Trojan is within the TrueType parsing engine, the same component it last patched just last month.

In a security advisory published yesterday, Microsoft spelled out some information about the vulnerability and said it was working on a patch. The company did not set a timetable for releasing the fix, but did say that the patch would not appear as part of the batch it plans to ship next week.

"We plan to release the security update through our security bulletin process, although it will not be ready for this month's bulletin release," said Jerry Bryant, a group manager with Microsoft's Trustworthy Computing Group, in a blog post late Thursday.

Microsoft has slated a four-patch update for Nov. 8.

According to Symantec, which has been working with the Laboratory of Cryptography and Systems Security (Crysys) at Budapest University, Duqu infects Windows PCs using a malformed Word document that's delivered to victims as an email attachment. When victims open the attached document, the exploit is triggered, giving Duqu's installer the foothold it needs to install the Trojan.

Duqu has been characterized by Symantec and others as a possible precursor to the next Stuxnet, the ultra-sophisticated worm that last year was pegged as an attack tool aimed at Iran's nuclear program.

As expected, Microsoft classified the kernel vulnerability as an elevation of privilege bug, meaning that it can be used to provide attackers with rights on the targeted PC necessary to install and run software.

Microsoft patched the same part of the "Win32k.sys" kernel-mode device driver just last month, when it fixed a flaw in the TrueType parsing engine that could let hackers conduct denial-of-service attacks to cripple Windows PCs.

That patch was one of four that patched various kernel bugs included in the MS11-077 security update.

Prior to last month, the most recent update to the TrueType parsing engine within Win32k.sys was in June 2010, when Microsoft issued MS10-032 to fix a different elevation of privilege bug.

Microsoft has been extremely busy patching pieces of the Windows kernel this year.

So far during 2011, Microsoft has patched 56 different kernel vulnerabilities with updates issued in February, April, June, July, August and October. In April alone, the company fixed 30 bugs, then quashed 15 more in July.

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

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

Blocking t2embed.dll, however, has side effects: Applications that rely on embedded fonts -- and those include not only Word but also many other programs, including browsers -- will not render text properly.

Microsoft hinted that it was unlikely to patch the latest kernel vulnerability with an emergency, or so-called "out-of-band," update unless the volume of attacks suddenly spikes.

"The risk for customers remains low," said Bryant. "[But] we are able to closely monitor the threat landscape and will notify customers if we see any indication of increased risk."

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 gkeizer@computerworld.com.

See more articles by Gregg Keizer.

Copyright © 2011 IDG Communications, Inc.

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