WTSSendMessageA "access is denied"

[T]

Hi All,

Windows Chromebook Edition (W11-Pro 22H2)

https://learn.microsoft.com/en-us/windows/win32/api/wtsapi32/nf-wtsapi32-wtssendmessagea

Rudy probably knows this one off the top of his head.

I have a weird one trying to use WTSSendMessageA. It
works perfectly from the command line as both
user and Administrator.

WTSSendMessageA(
$Handle,
$SessionId,
$pTitle,
$TitleLength,
$pMessage,
$MessageLength,
$Style,
$Timeout,
$pResponse,
$bWait );

But when I try to run it from the Task Scheduler
as Administrator, I get back: error 5 (GetLastError),
"Access is Denied" (nf-winbase-formatmessagew)

https://ibb.co/9nR9pcc

And to add insult to injury, The pop up you see
from ibb comes from a DLL a guy at M$ wrote me.
And that one works in Task Scheduler. AAAAHHHHHH!!!!

$Handle = WTS_CURRENT_SERVER_HANDLE;
$bWait = 1;

Yours in frustration,
-T

[R.Wieser]

T,

> https://learn.microsoft.com/en-us/windows/win32/api/wtsapi32/nf-wtsapi32-wtssendmessagea
>
> Rudy probably knows this one off the top of his head.

:-) Thanks for the confidence in me, but I'm afraid I know very little
about RDP in general.

> But when I try to run it from the Task Scheduler
> as Administrator, I get back: error 5 (GetLastError),
> "Access is Denied" (nf-winbase-formatmessagew)

...

> And to add insult to injury, The pop up you see
> from ibb comes from a DLL a guy at M$ wrote me.
> And that one works in Task Scheduler. AAAAHHHHHH!!!!

I read that as : I've got a program thats started by a remote task scheduler
which tries to open a message box at my end and fails, but I do get a
messagebox (showing the results of my failed attempt) when I use the M$ guys
DLL for it."

In that case my assumption is that that DLL does some more work than just
calling WTSSendMessage. Perhaps wrapping the call in a creation and
teardown of an actual RDP session (WTSVirtualChannelOpen and -close) ?

That the program works when you start it directly could be because the
environment for a program started thru the task scheduler is a bit different
from when its started by you in person. It by default not being
interactive is one of them.

Hope that helps.

Regards,
Rudy Wieser

[T]

On 11/19/22 00:34, R.Wieser wrote:
> T,
>
>> https://learn.microsoft.com/en-us/windows/win32/api/wtsapi32/nf-wtsapi32-wtssendmessagea
>>
>> Rudy probably knows this one off the top of his head.
>
> :-) Thanks for the confidence in me, but I'm afraid I know very little
> about RDP in general.

But, But, But, You know all and see all!!!!

>
>> But when I try to run it from the Task Scheduler
>> as Administrator, I get back: error 5 (GetLastError),
>> "Access is Denied" (nf-winbase-formatmessagew)
> ...
>> And to add insult to injury, The pop up you see
>> from ibb comes from a DLL a guy at M$ wrote me.
>> And that one works in Task Scheduler. AAAAHHHHHH!!!!
>
> I read that as : I've got a program thats started by a remote task scheduler
> which tries to open a message box at my end and fails, but I do get a
> messagebox (showing the results of my failed attempt) when I use the M$ guys
> DLL for it."
>
> In that case my assumption is that that DLL does some more work than just
> calling WTSSendMessage. Perhaps wrapping the call in a creation and
> teardown of an actual RDP session (WTSVirtualChannelOpen and -close) ?
>
> That the program works when you start it directly could be because the
> environment for a program started thru the task scheduler is a bit different
> from when its started by you in person. It by default not being
> interactive is one of them.
>
> Hope that helps.
>
> Regards,
> Rudy Wieser
>
>

That is what I believe is happening.