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Flag: Tornado! Hurricane!

Created: Friday, April 14 2006 01:57.24 CDT

Modified: Friday, April 14 2006 01:58.41 CDT

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New Shell Extension CLSID Black List

Author: randori82

# Views: 3139

There's a new reg key to black list CLSIDs for Shell Extensions from the latest MS patch set(MS06-015).

Here's a bit of detail on the keys for Shell Extensions (http://support.microsoft.com/kb/216384/EN-US/):
HKLM:Software\Microsoft\Windows\CurrentVersion\Shell Extensions\Approved is the CLSID list of Shell Extensions that are approved to run.

This is compared against when 'EnforceShellExtensionSecurity' is set to 1 in HKLM:Software\Microsoft\Windows\CurrentVersion\Policies\Explorer.  Nothing new there, that's existing functionality.

However, with MS06-015, there's a new key: HKLM:Software\Microsoft\Windows\CurrentVersion\Shell Extensions\Blocked that also gets referenced from ShouldLoadShellExt.

The *blocked list* is checked before the *allowed list*, so anything that is blocked will not run, whether or not it's in the *allowed list*.

So, following MS06-015, you can make sure hosts will not run Shell Extensions by adding their CLSID to the *blocked list*.

Obviously this couldn't be put in the workaround for this patch, but for future Shell Extension vulnerabilities, this is definatley a workaround.

This new key was found while reversing MS06-015 for XP.

~Andre Protas

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