Exploit the possiblities

7-Zip DLL Hijack

7-Zip DLL Hijack
Posted Dec 10, 2015
Authored by Stefan Kanthak

7-Zip executable installers suffer from a DLL hijacking vulnerability.

tags | exploit
systems | windows
MD5 | 7e6235971a640f786b8465b4f378a0a2

7-Zip DLL Hijack

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Hi @ll,

the executable installers [°] of 7-Zip (see <http://www.7-zip.org/>)
and ALL self-extracting archives created with 7-Zip 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 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
UXTheme.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 store
it as UXTheme.dll in your "Downloads" directory;

Note: this is the 32-bit DLL; the 64-bit DLL is available in
<http://home.arcor.de/skanthak/download/SENTINEL.CAB>

2. download <http://www.7-zip.org/a/7z1512.exe> and store it in the
"Downloads" directory;

3. run 7z1512.exe from the "Downloads" directory;

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


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 author

NO ANSWER, not even an acknowledgement of receipt

2015-12-05 vulnerability report resent to author

2015-12-05 response from author:
"What about another exe installers?
Firefox, Chrome, Skype, WinRAR and others.
All of them use exe installers."

2015-12-05 other executable installers don't matter here; see
but <https://bugzilla.mozilla.org/show_bug.cgi?id=792106> and
<https://www.mozilla.org/en-US/security/advisories/mfsa2012-98/>

2015-12-06 several more lame and COMPLETELY clueless responses
from author showing that he didn't even read the
sources referenced here

2015-12-08 report published



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