Beyond good ol’ Run key, Part 21

January 3, 2015 in Anti-Forensics, Autostart (Persistence), Compromise Detection, Forensic Analysis, Malware Analysis

MFC localization DLLs are similar in nature to Visual Basic ones (I talked about them in my last post).

The MFC framework went through many versions over the years, but the localization piece is still there and for many years it was initialized incorrectly by loading the resource DLLs as actual code (using LoadLibrary API). It eventually got fixed in the mfc110.dll (now it is using LoadLibraryEx API and forces to load the resource DLLs as data only), so while the new versions are secure the older versions can be potentially leveraged as a persistence mechanism.

As mentioned, there are many versions of MFC DLLs (refer to wiki link I provided in the first sentence of this post) so let’s walk through them one by one.

  • mfc40.dll
    • Loads MFC40LOC.dll
  • mfc42.dll
    • Loads MFC42LOC.dll
  • mfc70.dll
    • Loads MFC70<LNG>.dll & MFC70LOC.dll
  • mfc71.dll
    • Loads MFC71<LNG>.dll & MFC71LOC.dll
  • mfc80.dll
    • Loads MFC80<LNG>.dll & MFC80LOC.dll
  • mfc90.dll
    • Loads MFC90<LNG>.dll & MFC90LOC.dll
  • mfc100.dll
    • Loads MFC100<LNG>.dll & MFC100LOC.dll
    • Uses LoadLibraryEx, yet still loads the resource DLLs as code
    • Note: often accompanied by existing resource files for major languages: mfc100chs.dll, mfc100cht.dll, mfc100deu.dll, mfc100enu.dll, mfc100esn.dll, mfc100fra.dll, mfc100ita.dll, mfc100jpn.dll, mfc100kor.dll, mfc100rus.dll
  • mfc110.dll
    • Loads MFC110<LNG>.dll & MFC110LOC.dll
    • Note: often accompanied by existing resource files for major languages: mfc110chs.dll, mfc110cht.dll, mfc110deu.dll, mfc110enu.dll, mfc110esn.dll, mfc110fra.dll, mfc110ita.dll, mfc110jpn.dll, mfc110kor.dll, mfc110rus.dll
    • Note: This is the first version of MFC library that loads the resource DLL in a proper, secure way, via LoadLibraryEx using a LOAD_LIBRARY_AS_DATAFILE flag – as such, it doesn’t execute any code (that is, the persistence mechanism described here won’t work)
  • mfc120.dll
    • Loads MFC120<LNG>.dll & MFC120LOC.dll, but also loads them as data so persistence mechanism won’t work either
    • Note: often accompanied by existing resource files for major languages: mfc120chs.dll, mfc120cht.dll, mfc120deu.dll, mfc120enu.dll, mfc120esn.dll, mfc120fra.dll, mfc120ita.dll, mfc120jpn.dll, mfc120kor.dll, mfc120rus.dll

Note #1:

  • LNG is a 3-letter language code: ENU, CHS, JPN, FRA, etc. (e.g. MFC70ENU.dll)

Note #2:

  • The newer versions (>=70) of MFC load MFCxxxLOC.dll DLLs only if the LCID is set to LOCALE_SYSTEM_DEFAULT – since the loading of localization DLLs happens in a loop (all LCIDs are enumerated until loading of a localized DLLs succeeds), one of the enumerated LCIDs is actually set to LOCALE_SYSTEM_DEFAULT on Windows XP and both MFCxxx<LNG>.dll and MFCxxxLOC.dll are loaded, but I have not encountered the situation during tests where it would be set on Windows 7 (so you won’t see loading of MFCxxxLOC.dll on win7; this requires more research)

Note #3:

  • The Unicode and Debug versions of MFC libraries are behaving in an identical way e.g. for mfc110.dll you have:
    • mfc110.dll – ANSI
    • mfc110d.dll – ANSI, Debug
    • mfc110u.dll – Unicode
    • mfc110ud.dll – Unicode, Debug

To test, you can load the mfc DLL into OllyDbg, put a breakpoint on LoadLibraryA/W (and LoadLibraryExA/W for newer versions, and A/W for Ansi or Unicode respectively) and see what happens – the below screenshot shows various versions of MFC DLL loading the localization DLLs – first five are from English OS and the last two from Russian:


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