Defense in depth -- the Microsoft way

["Stefan Kanthak" <stefan.kanthak () nexgo de>]

Hi @ll,

the "Microsoft Installer" creates for applications installed via an
.MSI the following uninstall information in the Windows registry
(see <http://msdn.microsoft.com/library/aa372105.aspx>):

[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall]
"UninstallString"="MsiExec.Exe /X{<GUID>}"
"ModifyPath"="MsiExec.Exe /I{<GUID>}"

Note the unqualified path to the executable "msiexec.exe".

On Windows installations without the "SafeProcessSearchMode" hotfix
(cf. <http://support.microsoft.com/kb/905890>) or with this safeguard
turned off (cf. <http://msdn.microsoft.com/library/dd266735.aspx>,
which refers to <http://support.microsoft.com/kb/959426> alias MS09-015),
an executable "msiexec.exe" placed in the CWD or the users "base"
directory (addressed by "%HOMEDRIVE%%HOMEPATH%" and typically equal to
"%USERPROFILE%") can be run instead of the intended executable
"%SystemRoot%\System32\MsiExec.Exe".


The VERY simple fix (which eliminates this attack vector completely):
always use fully-qualified paths to the well-known executables.

JFTR: cf. <http://seclists.org/fulldisclosure/2011/Sep/160>

Stefan Kanthak

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