Hi @ll,

the executable (un)installers for Flash Player before version
22.0.0.192 and 18.0.0.360 (both released on 2016-06-15) are
vulnerable to DLL hijacking: they load and execute multiple
Windows system DLLs from their "application directory" instead
of Windows' "system directory" %SystemRoot%\System32\.

On Windows 7 and before they also (try to) load PCACli.dll and
API-MS-Win-Downlevel-Shell32-l1-1-0.dll from the PATH:
PCACli.Dll and API-MS-Win-Downlevel-Shell32-l1-1-0.dll are not
present there, these DLLs were first shipped with Windows 8.

On Windows XP and before they additionally try to load DWMAPI.dll,
PropSys.dll, DevRtl.dll and RPCRTRemote.dll from the PATH: these
DLLs were first shipped with Windows Vista.


See <https://cwe.mitre.org/data/definitions/426.html>,
<https://cwe.mitre.org/data/definitions/427.html> and
<https://capec.mitre.org/data/definitions/471.html> for details
about this well-known and well-documented beginner's error!


Due to the application manifest embedded in the executables which
specifies "requireAdministrator" the installers are run with
administrative privileges ("protected" administrators are prompted
for consent, unprivileged standard users are prompted for an
administrator password); execution of the DLLs therefore results
in an escalation of privilege!


Proof of concept/demonstration:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. visit (and read) <https://home.arcor.de/skanthak/sentinel.html>,
   then download <https://home.arcor.de/skanthak/download/SENTINEL.DLL>
   and save it as PCACli.dll, API-MS-Win-Downlevel-Shell32-l1-1-0.dll,
   DWMAPI.dll, RPCRTRemote.dll, OLEAcc.dll, PSAPI.dll, SetupAPI.dll,
   ClbCatQ.dll, WSock32.dll, WS2_32.dll, HNetCfg.dll, DNSAPI.dll,
   IPHlpAPI.dll, RASAPI32.dll, SensAPI.dll, RASAdHlp.dll, RASMan.dll
   plus UserEnv.dll, COMRes.dll, WS2Help.dll, TAPI32.dll, RTUtils.dll
   SAMLib.dll and WinMM.dll in your "Downloads" directory;

2. fetch the (un)installers for Flash Player released before 2016-06-15
   from Adobe's web site and save them in your "Downloads" directory;

3. run the (un)installers downloaded in step 2 and notice the message
   boxes displayed from the DLLs placed in step 1.

PWNED!


JFTR: since the (un)installers are 32-bit programs and (un)install
      both the 32-bit and 64-bit versions of Flash Player this POC
      works on 64-bit Windows too.


stay tuned
Stefan Kanthak


Timeline:
~~~~~~~~~

2016-03-12    first vulnerability report sent to Adobe

2016-03-13    Adobe acknowledged the receipt

2016-04-06    Adobe informed about the upcoming patch to be released
              2016-04-07 and the assignment of CVE-2016-1014

2016-04-17    second vulnerability report sent to Adobe: the "fixed"
              (un)installers are still vulnerable, they just load
              some other DLLs now

2016-04-20    Adobe confirmed the second report and announced to fix
              the vulnerability in the June update

2016-06-15    Adobe released fixed (un)installers

2016-06-17    report published