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Microsoft today turned to a new defensive measure to help users ward off ongoing attacks exploiting a known bug in Internet Explorer (IE).

As expected, the company also patched three vulnerabilities in Windows.

"The 'shim' for IE is the news today," said Andrew Storms, director of security operations at nCircle Security. "We had not expected a patch for IE today, but we had not expected the shim either."

Shim is a term used to describe an application compatibility workaround. Storms said it was appropriate to today's temporary fix because Microsoft used the Windows Application Compatibility Toolkit to modify IE so it's immune to attacks that leverage a bug in how the browser processes a CSS (Cascading Style Sheets) file.

"This is the first time that they've used the Application Compatibility Toolkit to mitigate a zero-day vulnerability," said Storms.

The tool, which has been part of Windows since XP's days, was designed to allow older applications, including those created for outdated versions of the OS, to run on newer editions of the operating system.

Microsoft's workaround used the Application Compatibility Toolkit to modify the core library of IE -- a DLL, or Dynamic-Link library, named "Mshtml.dll," that contains the rendering engine -- in memory each time IE runs. The modification prevents recursive loading of a CSS, which effectively stops the existing attacks.

"Microsoft's using the App Comp in an unexpected way," said Storms. "They've incarnated it to help mitigate a zero-day. They're going to use whatever is in their arsenal, that's the bottom line."

Other researchers also applauded the new tactic.


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