Date: Mon, 2 Jan 2017 12:53:28 +0100
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: <bugtraq@...urityfocus.com>
Cc: fulldisclosure@...lists.org
Subject: [FD] Executable installers are vulnerable^WEVIL (case 45):
	ReadPDF's installers allow escalation of privilege

Hi @ll,

the executable installer "InstallTinyPDF.exe", available from
<http://tinypdf.com/downloads.html>, is (surprise.-) vulnerable:

1. DLL hijacking (this is well-known and well-documented; 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>)

   "InstallTinyPDF.exe" loads and executes UXTheme.dll and/or
   DWMAPI.dll from its "application directory" instead Windows'
   "system directory", resulting in arbitrary code execution".

   The "application directory" is typically the user's "Downloads"
   directory; see
   <https://insights.sei.cmu.edu/cert/2008/09/carpet-bombing-and-directory-poisoning.html>
   and <http://blog.acrossecurity.com/2012/02/downloads-folder-binary-planting.html>

   If UXTheme.dll or DWMAPI.dll get planted for example per
   "drive-by download" in the users "Downloads" directory this
   vulnerability becomes a remote code execution.

2. Unsafe TEMP directory (this is well-known and well-documented;
   see <https://cwe.mitre.org/data/definitions/377.html>
   and <https://cwe.mitre.org/data/definitions/379.html>)

   "InstallTinyPDF.exe" creates a temporary (sub)directory
   "%TEMP%\is-<random>.tmp\" into which it extracts embedded DLLs
   (_ShFolder.dll [*], ...) and an embedded subinstaller (is-*.tmp)
   which it executes then ELEVATED, i.e. with administrative
   privileges.

   Since the temporary (sub)directory is unprotected (it inherits
   the NTFS ACL from its parent "%TEMP%\") the extracted executables
   can be overwritten the between their creation and execution by
   the unprivileged user, resulting in arbitryry code execution with
   escalation of privilege.

   Additionally (really: of course) "%TEMP%\is-<random>.tmp\is*-tmp"
   is vulnerable to DLL hijacking too: it loads UXTheme.dll and/or
   DWMAPI.dll and some more DLLs from its "application directory"
   "%TEMP%\is-<random>.tmp\", which can be placed there by the
   unprivileged user, again resulting in arbitrary code execution
   with elevation of privilege.


JFTR: InstallTinyPDF.exe is built using the crapware InnoSetup!


Mitigations:
~~~~~~~~~~~~

* Don't use executable installers! NEVER!
  Don't use self-extractors! NEVER!

  See <http://seclists.org/fulldisclosure/2015/Nov/101> and
  <http://seclists.org/fulldisclosure/2015/Dec/86> plus
  <http://home.arcor.de/skanthak/!execute.html> alias
  <https://skanthak.homepage.t-online.de/!execute.html> for more
  information.

* Practice STRICT privilege separation: NEVER use the so-called
  "protected" administrator account(s) created during Windows
  setup which use the same "%TEMP%" for unprivileged and privileged
  processes!

* Add an ACE "(D;OIIO;WP;;;WD)" to the ACL of every "%USERPROFILE%";
  use <https://msdn.microsoft.com/en-us/library/aa374928.aspx> to
  decode it to "deny execution of files in this directory for
  everyone, inheritable to all files in all subdirectories".


Fix:
~~~~

Write a windows-conformant .INF to install the printer driver
(see <http://home.arcor.de/skanthak/download/TINYPDF.INF>), then
use MakeCAT.exe to create a .CAT and SignTool.exe to sign it.

See <https://msdn.microsoft.com/en-us/library/aa386967.aspx>
and <https://msdn.microsoft.com/en-us/library/aa387764.aspx>
plus <https://msdn.microsoft.com/en-us/library/ff551778.aspx>
for the description of these development tools.

With this TINYPDF.INF the printer driver can be installed via
"Add Printer" as well as the following command lines:

* "%SystemRoot%\System32\RunDLL32.exe" "%SystemRoot%\System32\PrintUI.dll",PrintUIEntry /if /f "<path>\TINYPDF.INF" /m TinyPDF /r
LPT3:
  (see <https://technet.microsoft.com/en-us/library/ee624057.aspx>)

* DISM.exe /Image:<path> /Add-Driver /Driver:"<path>\TINYPDF.INF" ...
  (see <https://technet.microsoft.com/en-us/library/dd744355.aspx>)

* DPInst.exe ... which I but DON'T recommend!
  (see <https://msdn.microsoft.com/en-us/library/ms791049.aspx>)

Even Windows' setup can import it automatically from \$WinPEDriver$\
into the driver store (see
<https://technet.microsoft.com/en-us/library/cc766142.aspx>).


JFTR: there is ABSOLUTELY no need for executable installers on
      Windows! DUMP THIS CRAP!


stay tuned
Stefan Kanthak


[*] ShFolder.dll is cruft from the last millennium, it was used
    on Windows 9x without Internet Explorer 4; see
    <https://support.microsoft.com/en-us/kb/241733>
    Since Windows 2000 there is ABSOLUTELY no need to use this
    cruft!


Timeline:
~~~~~~~~~

2012-05-24    vulnerability report sent to vendor (yes, 2012-05-24!)

              no reply, not even an acknowledgement of receipt

2016-06-06    vulnerability report sent to vendor

              no reply, not even an acknowledgement of receipt

2017-01-02    report published

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