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UAC blocks windows messages

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  #1 (permalink)  
Old 05-30-2007

Posts: n/a
UAC blocks windows messages

We have a system that is comprised of 3 executables. They're all about
controlling displays.
1) Service.exe -- A service (running as local system)
2) Main.exe --The main app which is run as as local system too, but
within the interactive user's Session and attached to the user's
input desktop.
3) UI.exe -- A UI executable which runs under the interactive user's

Service launches Main.
Main launches UI, using CreateProcessAsUser(), getting the

UI.exe tries to broadcast WM_DISPLAYCHANGE at certain points which
needs to be heard by Main.exe.
But Main.exe's listening window & thread do not receive that message
if control-panel's UAC is checked on.
(BTW: On XP & win2k all is fine, and also on Vista when UAC=Off.)

Strangely calling ChangeDisplaySettingsEx() in the UI will
successfully cause WM_DISPLAYCHANGE to be heard by main.exe

Using Spy I can see that, for example, the frame of Notepad.exe does
receive the WM_DISPLAYCHANGE message, even though the window-procedure
in Main.exe does not.

I've tried using SendMessage, PostMessage, SendNotifyMessage, all
aimed at HWND_BROADCAST. All suffer the above problem under UAC.
I can add more details on how the processes are created if needs be.

Any suggestions of what's going on, and how I can fix it?
Thanks for any advice. I've been looking at this for a week now.

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  #2 (permalink)  
Old 05-31-2007

Posts: n/a
Re: UAC blocks windows messages
On 30 May, 17:56, Martin <> wrote:

I'll begin to answer my own question, for the benefit of anyone else
baffled by this.

It seems that UIPI (User Interface Privilege Isolation) is the cause.
In summary: a lower privilege process cant send windows-messages to
higher privilege processes.

Its defined in the document: "WindowsVistaUACDevReqs.doc", which can
be downloaded from a link on:

I'd still like any advice on a possible fix.
Is it possible to have the higher privilege process (Main.exe) that
launches the lower privilege process (UI.exe) open up a deliberate
chink in this UIPI armor?

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  #3 (permalink)  
Old 05-31-2007

Posts: n/a
Re: UAC blocks windows messages
OK, I got there in the end. :-)
Sorry for the noise.
The answer appears to be call ChangeWindowMessageFilter().

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  #4 (permalink)  
Old 05-31-2007
Jimmy Brush

Posts: n/a
Re: UAC blocks windows messages
On 31 May 2007 04:56:28 -0700, Martin <> wrote:

>OK, I got there in the end. :-)
>Sorry for the noise.
>The answer appears to be call ChangeWindowMessageFilter().

You may already know this, but running interactive programs on the
user's desktop running as system is considered a bad thing, even with
Vista's new security.

I'm sure you have a reason for doing so, but it would be better from a
security perspective if all the privileged code was running in your
service, and the only programs running on the users desktop was
running in the context of their user account.

- JB
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