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T1138 - Application Shimming

Description from ATT&CK

The Microsoft Windows Application Compatibility Infrastructure/Framework (Application Shim) was created to allow backward compatibility of programs as Windows updates and changes its code. For example, the application shimming feature allows developers to apply fixes to applications (without rewriting code) that were created for Windows XP so that it will work with Windows 10. (Citation: Engame Process Injection July 2017) Within the framework, shims are created to act as a buffer between the program (or more specifically, the Import Address Table) and the Windows OS. When a program is executed, the shim cache is referenced to determine if the program requires the use of the shim database (.sdb). If so, the shim database uses Hooking to redirect the code as necessary in order to communicate with the OS. A list of all shims currently installed by the default Windows installer (sdbinst.exe) is kept in:
  • %WINDIR%\AppPatch\sysmain.sdb
  • hklm\software\microsoft\windows nt\currentversion\appcompatflags\installedsdb

Custom databases are stored in:

  • %WINDIR%\AppPatch\custom & %WINDIR%\AppPatch\AppPatch64\Custom
  • hklm\software\microsoft\windows nt\currentversion\appcompatflags\custom

To keep shims secure, Windows designed them to run in user mode so they cannot modify the kernel and you must have administrator privileges to install a shim. However, certain shims can be used to Bypass User Account Control (UAC) (RedirectEXE), inject DLLs into processes (InjectDLL), disable Data Execution Prevention (DisableNX) and Structure Exception Handling (DisableSEH), and intercept memory addresses (GetProcAddress). Similar to Hooking, utilizing these shims may allow an adversary to perform several malicious acts such as elevate privileges, install backdoors, disable defenses like Windows Defender, etc.

Detection: There are several public tools available that will detect shims that are currently available (Citation: Black Hat 2015 App Shim):

  • Shim-Process-Scanner - checks memory of every running process for any Shim flags
  • Shim-Detector-Lite - detects installation of custom shim databases
  • Shim-Guard - monitors registry for any shim installations
  • ShimScanner - forensic tool to find active shims in memory
  • ShimCacheMem - Volatility plug-in that pulls shim cache from memory (note: shims are only cached after reboot)

Monitor process execution for sdbinst.exe and command-line arguments for potential indications of application shim abuse.

Platforms: Windows

Data Sources: Loaded DLLs, System calls, Windows Registry, Process Monitoring, Process command-line parameters

Permissions Required: Administrator

Atomic Tests


Atomic Test #1 - Application Shim Installation

This test injects a DLL into a custom application

Supported Platforms: Windows

Run it with command_prompt!

sdbinst.exe AtomicShimx86.sdb