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Unloading the Sysmon Minifilter Driver

✇Blog of Osanda
Osanda Malith Jayathissa

The binary fltMC.exe is used to manage minifilter drivers. You can easily load and unload minifilters using this binary. To unload the Sysmon driver you can use:

fltMC unload SysmonDrv

If this binary is flagged, we can unload the minifilter driver by calling the ‘FilterUnload’ which is the Win32 equivalent of ‘FltUnloadFilter’. It will call the minifilter’s ‘FilterUnloadCallback’ (PFLT_FILTER_UNLOAD_CALLBACK) routine. This is as same as using fltMC which is a Non-mandatory unload.
For calling this API SeLoadDriverPrivilege is required. To obtain this privelege adminsitrative permissions are required.

Here’s a simple C code I wrote to call the ‘FilterUnload’ API.

https://github.com/OsandaMalith/WindowsInternals/blob/master/Unload_Minifilter.c

[gist https://gist.github.com/OsandaMalith/3315bc640ff51227ab067052bc20a445]

Note that when unloading a minifilter driver by the FilterManager, it will be logged under the System log.

References:
https://www.osr.com/nt-insider/2017-issue2/introduction-standard-isolation-minifilters/

Unloading the Sysmon Minifilter Driver

✇Blog of Osanda
Osanda Malith Jayathissa

The binary fltMC.exe is used to manage minifilter drivers. You can easily load and unload minifilters using this binary. To unload the Sysmon driver you can use:

fltMC unload SysmonDrv

If this binary is flagged, we can unload the minifilter driver by calling the ‘FilterUnload’ which is the Win32 equivalent of ‘FltUnloadFilter’. It will call the minifilter’s ‘FilterUnloadCallback’ (PFLT_FILTER_UNLOAD_CALLBACK) routine. This is as same as using fltMC which is a Non-mandatory unload.
For calling this API SeLoadDriverPrivilege is required. To obtain this privelege adminsitrative permissions are required.

Here’s a simple C code I wrote to call the ‘FilterUnload’ API.

https://github.com/OsandaMalith/WindowsInternals/blob/master/Unload_Minifilter.c

#include "stdafx.h"
#include <Windows.h>
#include <fltuser.h>
#pragma comment(lib,"FltLib.lib")
/*
Author: Osanda Malith Jayathissa (@OsandaMalith)
Website: https://osandamalith.com
Description: Unloading a minifilter driver by calling the FilterUnload which is the Win32 equivalent of FltUnloadFilter.
It will call the minifilter's FilterUnloadCallback (PFLT_FILTER_UNLOAD_CALLBACK) routine.
In this code we are unloading the "SysmonDrv" minifilter.
You need administrative privs to escalete to SeLoadDriverPrivilege.
*/
typedef NTSTATUS(WINAPI *_RtlAdjustPrivilege)(
ULONG Privilege, BOOL Enable,
BOOL CurrentThread, PULONG Enabled);
int _tmain(int argc, _TCHAR* argv[]) {
ULONG t;
HRESULT unload;
LPCWSTR driver = L"SysmonDrv";
_RtlAdjustPrivilege RtlAdjustPrivilege = (_RtlAdjustPrivilege)GetProcAddress(GetModuleHandle(L"ntdll"), "RtlAdjustPrivilege");
RtlAdjustPrivilege(012, TRUE, FALSE, &t);
unload = FilterUnload(driver);
wprintf(L"%ls", unload == S_OK ?
L"Minifilter Successfully Unloaded" :
L"An Error Occured. Check Privs."
);
return 0;
}

Note that when unloading a minifilter driver by the FilterManager, it will be logged under the System log.

References:
https://www.osr.com/nt-insider/2017-issue2/introduction-standard-isolation-minifilters/

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