why is window hook blocked from calling SendMessage

I'm having problems with communicating between a window called from hook and my main app using SendMessage.
The code is basically a global hook defined as:

hookCallWndProc = SetWindowsHookEx(WH_CALLWNDPROC, CallWndProc, ModuleFromAddress(CallWndProc), 0);

and when event occurs globally the callback function CallWndProc is called from which I use SendMessage(callbackWindowHandle, 12345, NULL, NULL);
and the message is received on my message-only window under 12345 code for all windows except firefox.
but the code in CallWndProc for firefox gets executed also because I tested it by calling SetWindowPos and was able to change the position of window.

LRESULT CALLBACK CallWndProc(int code, WPARAM wparam, LPARAM lparam) 
{ 	
	if (code >= 0) 	
	{
 		CWPSTRUCT* pCwpStruct = (CWPSTRUCT*)lparam;
 		switch (pCwpStruct->message)
 		{ 		case WM_WINDOWPOSCHANGED:
 		{ 			WINDOWPOS* pWindowPosStruct = (WINDOWPOS*)pCwpStruct->lParam;
 			SendMessage(callbackWindowHandle, 12345, NULL, NULL);
 			SetWindowPos(pCwpStruct->hwnd, NULL, 600, 200, 0, 0, SWP_NOSIZE | SWP_NOZORDER | SWP_NOREDRAW | SWP_NOACTIVATE | SWP_NOOWNERZORDER | SWP_NOSENDCHANGING);
 			break;
 		}
 		}
 	}
 	return CallNextHookEx(hookCallWndProc, code, wparam, lparam); }

Another hook that doesnt work for firefox window is global CBT hook:
hookCbt = SetWindowsHookEx(WH_CBT, CBTProc, ModuleFromAddress(CBTProc), 0);
where CBTProc is defined:

LRESULT CALLBACK CBTProc(int nCode, WPARAM wParam, LPARAM lParam)
{
	switch (nCode)
	{
	case HCBT_ACTIVATE:
		SendMessage(callbackWindowHandle, 12346, NULL, NULL);
		break;
	}
	return CallNextHookEx(hookCbt, nCode, wParam, lParam);
}

why does SendMessage function call not work for firefox window? Is there any other way to communicate with another process from hook callback beside SendMessage?

I tried subclassing firefox window which succeeds but SendMessage/PostMessage/NotifyMessage don't work.

> Solution was to use ChangeWindowMessageFilterEx and pass 12345 for message parameter and MSGFLT_ALLOW for action parameter, presumably firefox filters all messages but ones it uses and to be sent out from its process space (or window).

Your solution is correct, but your reasoning is not.

FireFox (and other web browsers) are typically run as low-integrity processes, for security/safety purposes (ie, to sandbox the browser to protect your machine from unwanted Internet dangers).

As such, when your hook runs inside of a Firefox (or other web browser) process, and it tries to send out a window message to a higher-integrity process, such as your main app, UIPI blocks the message by default. Which you would have known had your code been checking SendMessage() for failure, per its documentation:

> Message sending is subject to UIPI. The thread of a process can send messages only to message queues of threads in processes of lesser or equal integrity level.
>
> ...
>
> When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).

Calling ChangeWindowMessageFilterEx() inside of your app lets you tell UIPI to allow your custom message to come through from lower-integrity processes, like FireFox.