T1546.011: Application Shimming
Adversaries may establish persistence and/or elevate privileges by executing malicious content triggered by application shims. The Microsoft Windows Application Compatibility Infrastructure/Framework (Application Shim) was created to allow for backward compatibility of software as the operating system codebase changes over time. 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.
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
andhklm\software\microsoft\windows nt\currentversion\appcompatflags\installedsdb
Custom databases are stored in:
%WINDIR%\AppPatch\custom & %WINDIR%\AppPatch\AppPatch64\Custom
andhklm\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 and RedirectEXE), inject DLLs into processes (InjectDLL), disable Data Execution Prevention (DisableNX) and Structure Exception Handling (DisableSEH), and intercept memory addresses (GetProcAddress).
Utilizing these shims may allow an adversary to perform several malicious acts such as elevate privileges, install backdoors, disable defenses like Windows Defender, etc. Shims can also be abused to establish persistence by continuously being invoked by affected programs.
Positive Technologies products that cover the technique
Description of detection methods is not available yet
Detection
ID | DS0011 | Data source and component | Module: Module Load | Description | Monitor DLL loads by processes that load user32.dll and look for DLLs that are not recognized or not normally loaded into a process. |
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ID | DS0009 | Data source and component | Process: Process Creation | Description | Monitor newly executed processs for sdbinst.exe for potential indications of application shim abuse. There are several public tools available that will detect shims that are currently available :
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ID | DS0024 | Data source and component | Windows Registry: Windows Registry Key Modification | Description | Monitor for changes to windows registry keys and/or values that may establish persistence and/or elevate privileges by executing malicious content triggered by application shims. |
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ID | DS0017 | Data source and component | Command: Command Execution | Description | Monitor executed commands and arguments for sdbinst.exe for potential indications of application shim abuse. |
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ID | DS0022 | Data source and component | File: File Modification | Description | Monitor for changes made to files that may establish persistence and/or elevate privileges by executing malicious content triggered by application shims. |
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Mitigation
ID | M1052 | Name | User Account Control | Description | Changing UAC settings to "Always Notify" will give the user more visibility when UAC elevation is requested, however, this option will not be popular among users due to the constant UAC interruptions. |
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ID | M1051 | Name | Update Software | Description | Microsoft released an optional patch update - KB3045645 - that will remove the "auto-elevate" flag within the sdbinst.exe. This will prevent use of application shimming to bypass UAC. |
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