Monday, October 3, 2011

Windows Vulnerability Paradox

For those who read just the first few lines, this is not a critical vulnerability. It is low impact but interesting, so keep reading.
This post describes the Windows vulnerability I showed during my Black Hat USA 2011 workshop “Easy and Quick Vulnerability Hunting in Windows”.

The Windows security update for Visual C++ 2005 SP1 Redistributable Package (MS11-025) is a security patch for a binary planting vulnerability. This kind of vulnerability occurs when someone opens or executes a file and this file (or the application used to open the file) has dependencies (like DLL files) that will be loaded and executed from the current folder or other folders than can be attacker controlled. This particular vulnerability allows an attacker to execute arbitrary code by tricking a victim user into opening a file from a network share. When the victim user opens the file, the application associated with the file is executed, and an attacker-crafted DLL file is loaded and executed by the application.
It’s either funny or scary (you choose) that the Windows security update meant to fix the above-described vulnerability is also vulnerable to the same kind of vulnerability it fixes, and it can be exploited to elevate privileges.
When installing the security update on 64-bit Windows 7, the file vcredist_x64.exe is downloaded and then executed under the System account (the most powerful Windows account, it has full privileges) with some command line options:
C:\Windows\SoftwareDistribution\Download\Install\vcredist_x64.exe" /q:a /c:"msiexec /i vcredist.msi /qn
After being run, vcredist_x64.exe tries to launch the msiexec.exe process from the C:\Windows\Temp\IXP000.TMP\temporary folder, which is where the vcredist.msi used in the command line option is located, but because msiexec.exe doesn’t exist there, vcredist_x64.exe will fail to run it. Then vcredist_x64.exe launches msiexec.exefrom C:\Windows\SysWOW64, where msiexec.exe is located by default on 64-bit Windows 7.
There is an obvious vulnerability and it can be exploited by low-privilege Windows users since theC:\Windows\Temp\IXP000.TMP\ temporary folder DACL has write permissions to the Users group, so any Windows user can place in that temporary folder a file named msiexec.exe and execute arbitrary code under the System account when they attempt to install the vulnerable security update.
While this is an interesting vulnerability, it’s not critical at all. First, to be vulnerable you have to have the vulnerable package installed and without the security update applied. Second, for an attacker to exploit this vulnerability and elevate privileges, the option “Allow all users to install updates on this computer” must be enabled. This option is enabled on some systems, depending on configuration settings about how Windows updates are installed.
This presents an interesting paradox in that you're vulnerable if you haven’t applied the vulnerable patch and you're not vulnerable if you have applied the vulnerable patch. This means that the patch for the vulnerable patch is the vulnerable patch itself.
The following links provide some more technical details and video demonstrations about this vulnerability and how it can be exploited:

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