Microsoft Skype DLL Hijacking

2018.02.11
Risk: Medium
Local: Yes
Remote: No
CVE: N/A
CWE: N/A

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They've reviewed the code and were able to reproduce the issue, but have determined that the fix will be implemented in a newer version of the product rather than a security update. The team is planning on shipping a newer version of the client, and this current version will slowly be deprecated.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


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