Hi @ll,

since about two or three years now, Microsoft offers Skype as
optional update on Windows/Microsoft Update.

JFTR: for Microsoft's euphemistic use of "update" see
      <http://seclists.org/fulldisclosure/2018/Feb/17>

Once installed, Skype uses its own proprietary update mechanism
instead of Windows/Microsoft Update: Skype periodically runs
    "%ProgramFiles%\Skype\Updater\Updater.exe"
under the SYSTEM account.
When an update is available, Updater.exe copies/extracts another
executable as "%SystemRoot%\Temp\SKY<abcd>.tmp" and executes it
using the command line
    "%SystemRoot%\Temp\SKY<abcd>.tmp" /QUIET

This executable is vulnerable to DLL hijacking: it loads at least
UXTheme.dll from its application directory %SystemRoot%\Temp\
instead from Windows' system directory.

An unprivileged (local) user who is able to place UXTheme.dll or
any of the other DLLs loaded by the vulnerable executable in
%SystemRoot%\Temp\ gains escalation of privilege to the SYSTEM
account.


The attack vector is well-known and well-documented as CAPEC-471:
<https://capec.mitre.org/data/definitions/471.html>

Microsoft published plenty advice/guidance to avoid this beginner's
error: <https://msdn.microsoft.com/en-us/library/ff919712.aspx>,
<https://technet.microsoft.com/en-us/library/2269637.aspx>,
<https://support.microsoft.com/en-us/help/2389418/secure-loading-of-libraries-to-prevent-dll-preloading-attacks>
and
<https://blogs.technet.microsoft.com/srd/2014/05/13/load-library-safely/>
... which their own developers and their QA but seem to ignore!


See <https://bugs.chromium.org/p/project-zero/issues/detail?id=440>
for the same vulnerability in another Microsoft product!


stay tuned
Stefan Kanthak


Timeline:
~~~~~~~~~

2017-09-02    vulnerability report sent to vendor

2017-09-03    reply from vendor: "MSRC case 40550 opened"

2017-09-06    notification from vendor's case manager: "report passed
              to product group for investigation"

2017-10-27    reply from vendor's case manager:

              "The engineers provided me with an update on this case.
               They've reviewed the code and were able to reproduce
               the issue, but have determined that the fix will be
               implemented in a newer version of the product rather
               than a security update. The team is planning on shipping
               a newer version of the client, and this current version
               will slowly be deprecated. The installer would need a
               large code revision to prevent DLL injection, but all
               resources have been put toward development of the new
               client."

2018-02-09    report published