SilentClean UAC bypass via binary planting

This project implements a DLL planting technique to bypass UAC Always Notify and execute code in a high integrity process.

When SilentCleanup task is launched, dismhost searches for the non existing DLL api-ms-win-core-kernel32-legacy-l1.dll under:

C:\Users\USER\Appdata\Local\Microsoft\WindowsApps

The above path exists by default in the PATH of the user.

By crafting a malicious DLL and placing it in the above directory, it will be loaded by dismhost.exe and executed with High Integrity privileges.

Implementation

The project consists of:

• SilentClean .NET project - Launching SilentClean scheduled task with the use of the TaskScheduler library

• DLLmain_template.c - A DLL skeleton which will spawn a process and inject the shellcode of our choice. Sample provided implements a simple CreateRemoteThread injector.

• Cobalt strike aggressor script responsible for:

  • Generating the shellcode byte array
  • Replacing dllmain_template.c with the above shellcode
  • Compile the dll with mingw
  • Upload the dll to the required path
  • Execute .NET binary SilentClean.exe through Execute-Assembly to launch the scheduled task

Configuration

• Feel free to replace injection method in RunMe function of dllmain_template.c. This is just a POC
• Current spawned process to inject to is cmd.exe.
• No shellcode encryption / compression has been baked in. As such the DLL generated will probably be flagged by an AV
• x86_64-w64-mingw32 and headers are required to be installed on the building system
• If CNA can not find mingw replace the variables $mingwgcc $mingwdllwrap with your path
• Compile SilentClean .NET project and place executable in the same folder as the CNA script

Versions tested

• Microsoft Windows 10 - 1909 18363.1110
• Microsoft Windows 10 - 1909 18363.1082
• Microsoft Windows 10 - 1809 17763.1457

Author

• @leftp
• @cirrusj