Message-ID: <826672CE9F59441E984240E5EA64668F@W340>
Date: Tue, 8 Dec 2015 00:37:03 +0100
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: <fulldisclosure@...lists.org>
Cc: <bugtraq@...urityfocus.com>
Subject: Executable installers are vulnerable^WEVIL (case 5): JRSoft InnoSetup

Hi @ll,

executable installers [°] created with JRSoft InnoSetup
(see <http://jrsoftware.org/isinfo.php>; this includes of course
InnoSetup itself too) are vulnerable:

1. They load and execute a rogue/bogus/malicious UXTheme.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 UXTheme.dll gets 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
   UXTheme.dll then results in an escalation of privilege!

2. They extract embedded DLLs (_ShFolder.dll [²]) to an unsafe
   temporary (sub)directory "%TEMP%\is-<random>.tmp\" and load them
   from there [³].

   These DLLs can be overwritten by an unprivileged user between their
   creation and execution, resulting in an escalation of privilege.

3. They extract an embedded subinstaller (is-*.tmp) to an unsafe
   temporary (sub)directory "%TEMP%\is-<random>.tmp\" and load it
   from there [³].

   These EXEs can be overwritten by an unprivileged user between their
   creation and execution, resulting 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 UXTheme.dll in your "Downloads" directory;

2. download <http://jrsoftware.org/download.php/is.exe>
   (via <http://jrsoftware.org/isdl.php>) and save it in your
   "Downloads" directory;

3. execute isetup-5-5-6.exe from your "Downloads" directory;

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


Detection:
~~~~~~~~~~

Unless overwritten by the creator of the executable installer the
string "This installation was built with Inno Setup." contained in
their version resource and the string "JR.Inno.Setup" contained in
their embedded application manifest identify these executable installers.


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 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> and/or
   <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://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] 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>

    DONT USE the sample code shown in this MSKB article!

[³] Another well-known (trivial, easy to exploit and easy to avoid)
    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> ...


Timeline:
~~~~~~~~~

2015-11-28    vulnerability report sent to author

2015-12-01    reponse from author:
              "This is not a vulnerability."

2015-12-01    OUCH!
              You should have read the linked documents.
              For some prior "art" see:
              <https://bugzilla.mozilla.org/show_bug.cgi?id=811557>
              <http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-4833>
              <https://bugzilla.mozilla.org/show_bug.cgi?id=792106>
              <https://www.mozilla.org/en-US/security/advisories/mfsa2012-98/>
              <http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-4206>

              NO ANSWER, not even an acknowledgement of receipt

2015-12-08    report published

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