VLC DLL Hijack

2015-12-08T00:00:00
ID PACKETSTORM:134692
Type packetstorm
Reporter Stefan Kanthak
Modified 2015-12-08T00:00:00

Description

                                        
                                            `Hi @ll,  
  
the executable installers [°] of the videolan client (VLC, see  
<http://www.videolan.org/>) are vulnerable:  
  
1. They load and execute a rogue/bogus/malicious ShFolder.dll ['][²]  
(and other DLLs like SetupAPI.dll or UXTheme.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 ShFolder.dll (or any of the other DLLs) 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  
ShFolder.dll et. al. then results in an escalation of privilege!  
  
2. They extract embedded DLLs (System.dll, LangInfo.dll, UAC.dll, ...)  
to an unsafe temporary (sub)directory "%TEMP%\ns<letter><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. Their uninstaller copies itself to "%TEMP%\~nsu.tmp\<letter>u_.exe"  
and runs its copy from there, again loading  
"%TEMP%\~nsu.tmp\ShFolder.dll" and other DLLs, which can be created  
(in advance) or overwritten by an unprivileged user.  
  
Since "%TEMP%\~nsu.tmp\<letter>u_.exe" is typically started with  
administrative privileges this results in another 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 ShFolder.dll in your "Downloads" directory, then copy it  
as UXTheme.dll and SetupAPI.dll there too;  
  
2. download  
<http://get.videolan.org/vlc/2.2.1/win32/vlc-2.2.1-win32.exe>  
(via <http://www.videolan.org/> and save it in your "Downloads"  
directory;  
  
3. execute vlc-2.2.1-win32.exe from your "Downloads" directory;  
  
4. notice the message boxes displayed from ShFolder.dll etc. 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 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.  
  
['] 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!  
  
[²] 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>  
  
[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:  
~~~~~~~~~  
  
2015-12-06 vulnerability report sent to authors  
  
2015-12-07 response from author:  
"the issue is not in VLC, but in NSIS."  
  
I really LOVE this sort of $%§*@ attitude:  
"Once the rockets are up who cares where they come down,  
'That's not my department', says Wernher von Braun"  
  
2015-12-07 it's YOUR software, it's YOUR decision to use NSIS, it's  
YOUR (IR)responsibility!  
  
2015-12-07 report published  
`