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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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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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