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Executable installers are vulnerable^WEVIL (case 3): WiX Toolset's bootstrapper "burn.exe"



Hi @ll,

executable installers [°] created with the WiX Toolset (see
<http://wixtoolset.org/>, and of course the WiX Toolset installer
itself too) resp. using its bootstrapper "burn.exe" are vulnerable:
see <https://www.firegiant.com/blog/2016/1/20/wix-v3.10.2-released/>


1. They load and execute a rogue/bogus/malicious FEClient.dll [']
   (and other DLLs too, dependent on the version of Windows:
   MSI.dll, ClbCatQ.dll, Version.dll, ...) eventually found in the
   directory they are started from (the "application directory").

   For software downloaded with a web browser this is typically
   the "Downloads" directory: see
   <https://insights.sei.cmu.edu/cert/2008/09/carpet-bombing-and-directory-poisoning.html>,
   <http://blog.acrossecurity.com/2012/02/downloads-folder-binary-planting.html>
   and <http://seclists.org/fulldisclosure/2012/Aug/134>

   If FEClient.dll etc. get(s) planted in the "Downloads"
   directory per "drive-by download" this vulnerability becomes a
   remote code execution.

   Due to an application manifest embedded in the executable which
   specifies "requireAdministrator" or the "installer detection" (see
   <https://technet.microsoft.com/en-us/library/dd835540.aspx#BKMK_InstDet>)
   of Windows' "user account control" executable installers are
   typically started with administrative privileges ("protected"
   administrators are prompted for consent, unprivileged standard
   users are prompted for an administrator password); execution of
   FEClient.dll et. al. then results in an escalation of privilege!


2. They extract "WiXStdBA.dll" (and other files) to an unsafe
   temporary (sub)directory "%TEMP%\{<GUID>}\.b<letter>[<number>]\"
   and load it (resp. process them) from there [²].

   These files can be overwritten or compromised by an unprivileged
   user after extraction and before they are used.

   If such an executable installer is distributed per software
   deployment system (for example WSUS) it is run under "LocalSystem"
   account.

   Processes running under "LocalSystem" account use the global
   %TEMP% directory %SystemRoot%\Temp where EVERY (unprivileged)
   user can create (or overwrite) files and conduct an attack on
   "WiXStdBA.dll" and any other file created there.

   For a recent example of such a vulnerability see
   <https://code.google.com/p/google-security-research/issues/detail?id=440>


Proof of concept/demonstration:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. visit <http://home.arcor.de/skanthak/sentinel.html>, download
   <http://home.arcor.de/skanthak/download/SENTINEL.DLL> and save
   it as FEClient.dll in your "Downloads" directory, then copy it
   as MSI.dll, Version.dll, CLbCatQ.dll;

2. download
   <https://downloadmirror.intel.com/24345/a08/Intel%20Driver%20Update%20Utility%20Installer.exe>
   from <http://www.intel.com/p/de_DE/support/detect> resp.
   <https://downloadcenter.intel.com/download/24345/Intel-Driver-Update-Utility>
   and save it in your "Downloads" directory;

3. execute "Intel Driver Update Utility Installer.exe" from your
   "Downloads" directory;

4. notice the message boxes displayed from the DLLs placed in step 1.


Instead of Intel's Driver Update Utility installer you can use ANY
other executable installer built with WiX Toolset's bootstrapper
"burn.exe", for example Intel's Chipset Software installer, ...


Mitigation(s):
~~~~~~~~~~~~~~

0. DON'T USE EXECUTABLE INSTALLERS [°]!

   If your favourite applications are not distributed in the native
   installer package format of the resp. target platform: ask^WURGE
   their vendors/developers to provide native installation packages.
   If they don't: dump these applications, stay away from such cruft!

1. Turn off UAC's privilege elevation for standard users and installer
   detection for all users:

   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
   "ConsentPromptBehaviorUser"=dword:00000000 ; Automatically deny elevation requests
   "EnableInstallerDetection"=dword:00000000

   See <https://technet.microsoft.com/en-us/library/dd835564.aspx#BKMK_RegistryKeys>

2. NEVER execute files in UNSAFE directories (like "Downloads" and
   and "%TEMP%")!

3. Deny execution (at least) in the "Downloads" directories and all
   "%TEMP%" directories and their subdirectories:

   * Add the NTFS ACE "(D;OIIO;WP;;;WD)" meaning "deny execution of
     files in this directory for everyone, inheritable to all files
     in all subdirectories" (use CACLS.EXE /S:<SDDL> for example);

   * Use "software restriction policies" resp. AppLocker.

   Consider to apply either/both to every "%USERPROFILE%" as well as
   "%ALLUSERSPROFILE%" alias %ProgramData%" and "%PUBLIC%": Windows
   doesn't place executables in these directories and beyond.

   See <http://home.arcor.de/skanthak/safer.html> as well as
   <http://mechbgon.com/srp/> plus
   <http://csrc.nist.gov/itsec/SP800-68r1.pdf>,
   <https://www.nsa.gov/ia/_files/os/win2k/application_whitelisting_using_srp.pdf>
   or <https://books.google.de/books?isbn=1437914926> and finally
   <http://www.asd.gov.au/infosec/top35mitigationstrategies.htm>!


stay tuned
Stefan Kanthak


PS: This vulnerability results from the use of the Win32 API for
    the NTFS encrypting file system [³] (EFS) in "burn.exe".

    Due to a beginner's error made about 16 years ago during the
    development of EFS and still present in current versions of
    Windows this loads FEClient.dll with a simple (unqualified)
    filename ['] (see MS16-007 and CVE-2016-0014).


PPS: see <http://seclists.org/fulldisclosure/2015/Nov/101> and
     <http://seclists.org/fulldisclosure/2015/Dec/86> plus
     <http://home.arcor.de/skanthak/sentinel.html> and the not
     yet finished <http://home.arcor.de/skanthak/!execute.html>
     for more details!


PPPS: the case numbers are not in chronological order.


[°] Self-extracting archives and executable installers are flawed^W
    b(rainde)ad in concept and dangerous in practice.

    DON'T USE SUCH CRUFT!
    ALWAYS use the resp. target platforms native package and archive
    format.

    For Windows these are .INF (plus .CAB) and .MSI (plus .CAB),
    introduced 20 years ago (with Windows 95 and Windows NT4) resp.
    16 years ago (with Office 2000).

    Both .INF and .MSI are "opened" by programs residing in
    %SystemRoot%\System32\ which are therefore immune to this kind
    of "DLL and EXE Search Order Hijacking" attack.
    Since both .INF and .MSI access the contents of .CAB directly
    they eliminate the attack vector "unsafe temporary directory"
    too.

['] A well-known (trivial, easy to avoid, easy to detect, easy
    to exploit and easy to fix) and well-documented vulnerability:
    see <https://cwe.mitre.org/data/definitions/426.html>,
    <https://cwe.mitre.org/data/definitions/427.html>,
    <https://capec.mitre.org/data/definitions/471.html>,
    <https://technet.microsoft.com/en-us/library/2269637.aspx>,
    <https://msdn.microsoft.com/en-us/library/ff919712.aspx> and
    <https://msdn.microsoft.com/en-us/library/ms682586.aspx>

[²] Another well-known (trivial, easy to avoid, easy to detect, easy
    to exploit and easy to fix) and well-documented vulnerability:
    see <https://cwe.mitre.org/data/definitions/377.html>,
    <https://cwe.mitre.org/data/definitions/379.html>,
    <https://capec.mitre.org/data/definitions/27.html>,
    <https://capec.mitre.org/data/definitions/29.html> ...

[³] see <https://technet.microsoft.com/en-us/library/cc700811.aspx>