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Executable installers are vulnerable^WEVIL (case 6): SumatraPDF-*-installer.exe allows remote code execution with escalation of privilege



Hi @ll,

the executable installers [°] of all versions of SumatraPDF (see
<http://www.sumatrapdfreader.org/free-pdf-reader-de.html>) are
vulnerable:

1. On Windows Embedded POSReady 2009 (alias Windows XP SP3) the
   installer of the current version 3.1.1 loads and executes a
   rogue/bogus/malicious DCIMan32.dll ['] eventually found in the
   directory it is started from (the "application directory").

2. The installers of older versions of SumatraPDF load and execute
   a rogue/bogus/malicious UXTheme.dll ['] (and depending on the
   Windows version others like DCIMan32.dll too) 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 DCIMan32.dll, UXTheme.dll etc. get(s) planted in the users
   "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
   DCIMan32.dll then results in an escalation of privilege!


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 DCIMan32.dll in your "Downloads" directory;

2. download
   <https://kjkpub.s3.amazonaws.com/sumatrapdf/rel/SumatraPDF-3.1.1-install.exe>
   (via <http://www.sumatrapdfreader.org/free-pdf-reader-de.html>)
   or <http://software.zeniko.ch/sumatrapdf/SumatraPDF-install.exe>
   and save it in your "Downloads" directory;

3. execute SumatraPDF-3.1.1-install.exe from your "Downloads"
   directory;

4. notice the message box displayed from DCIMan32.dll placed in
   step 1;

5. in your "Downloads" directory copy DCIMan32.dll as UXTheme.dll;

6. download
   <http://kjkpub.s3.amazonaws.com/sumatrapdf/rel/SumatraPDF-3.0-install.exe>
   (via <http://www.sumatrapdfreader.org/download-prev-de.html>)
   and save it in your "Downloads" directory;

7. execute SumatraPDF-3.0-install.exe from your "Downloads" directory;

8. notice the message box displayed from DCIMan32.dll and UXTheme.dll
   placed in steps 1 and 5.


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: see <http://seclists.org/fulldisclosure/2015/Nov/101> (resp. the
    not yet finished <http://home.arcor.de/skanthak/!execute.html>)
    for more details!

PPS: 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 exploit and easy to avoid) and
    well-documented vulnerability: see
    <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>


Timeline:
~~~~~~~~~

2015-11-18    vulnerability report sent to authors

              NO ANSWER, not even an acknowledgement of receipt

2015-11-29    vulnerability report resent to authors

2015-11-29    response from author:
              "we don't load dciman32.dll in our code.
               [...]
               Either way, there's no bug in Sumatra"

2015-11-29    on Windows XP DCIMan32.dll is loaded due to the use
              of GDI32.dll. This is unique behaviour, not seen in
              any other executable installer I know.

              NO ANSWER, not even an acknowledgement of receipt

2015-12-07    report published