Date: Tue, 9 Mar 1999 12:57:42 -0800
From: Aleph One <aleph1@UNDERGROUND.ORG>
To: BUGTRAQ@netspace.org
Subject: Windows NT Screen Saver Vulnerability


Cybermedia Software has found the following vulnerability:

< http://www.cybermedia.co.in/NT%20Security/SS%20vulnerability.htm >


   Screen Saver vulnerability

   Description:

   The Screen Saver is started by Winlogon.Exe whenever the machine is
          idle for the specified amount of time. Screen Saver setting is
          a per user property and every user has right to set his own
          screen saver.

   The screen saver is started by Winlogon.Exe, initially in a suspended
          mode using CreateProcess API call. Once Winlogon.Exe gets the
          process handle to screen saver, it changes the primary security
          token of the screen saver to that of the logged in user and
          then resumes the screen saver process. This is done for
          security reasons. If Winlogon were to NOT do this, then screen
          saver would run with the security context of Winlogon.Exe
          (which runs in system context).



   Problem:

   The Winlogon.Exe DOES NOT check whether the changing of Primary token
          is successful. Hence if setting of primary token fails due to
          some reason, the screen saver binary will run in system context
          and be able to do whatever it pleases (e.g adding the logged in
          user to admin group).



   Simulation:

   On Windows NT 3.51 and all its service packs, Windows NT 4.0 with
          Service Pack 1, and NT 5.0 beta1 and beta2, when an MS-DOS
          application is spawned, the returned process handle is junk
          (rather it is a special event handle).

   The simulation consists of one 32-bit application say BEADMIN.EXE and
   one MS-DOS based application, say SCRNSAVE.EXE. The BEADMIN.EXE when
   started does the following
     * Creates one event in `not-signal'ed state
     * Sets up the screen saver. The screen saver executable is specified
       as SCRNSAVE.EXE and the timeout is set to minimum. . BEADMIN.EXE
       now waits on the event.

   After some time, the screen saver is triggered. This results in
   Winlogon.Exe spawning SCRNSAVE.EXE. Since the CreateProcess call
   returns junk handle to Winlogon.Exe, the setting of primary token
   fails. Hence the SCRNSAVE.EXE application (NTVDM.EXE) runs in System
   Context. This SCRNSAVE.EXE again spawns BEADMIN.EXE application. Now
   this second copy of BEADMIN.EXE inherits the security context of NTVDM
   which is System Context. This application adds the logged in user to
   admin group and signals the event on which first instance of
   BEADMIN.EXE is waiting. In response to this the first copy of
   BEADMIN.EXE resets back the Screen Saver settings and quits.

   The logged in user name is passed between the first and second copy of
   BEADMIN.EXE using shared section.

   Comments:

   Although this program does not run on versions of Windows NT 4.0 after
          Service pack 1, the vulnerability exists in these versions as
          well. i.e in these versions also Winlogon.exe fails to perform
          the validation. but the condition required for simulation does
          not happen. i.e In these versions, winlogon.exe gets the proper
          handle to the process.

   Since the vulnerability is once again reproducible in the beta
   versions of NT 5.0, it is clear that it needs to be fixed.

   [1]Download Demo for Screen Saver vulnerability

   Blueline.jpg (398 bytes)

    Copyright© 1999, Cybermedia Software Private Limited. All trademarks
                 are property of their respective holders.

References

   1. http://www.cybermedia.co.in/Free%20Downloads/ScrnSave.zip

-----------------------------------------------------------------------------

Date: Wed, 10 Mar 1999 19:10:55 -0700
From: Steve Manzuik <smanzuik@HOME.COM>
To: NTBUGTRAQ@LISTSERV.NTBUGTRAQ.COM
Subject: Re: NT SCREENSAVER VULNERABILITY

And if you read the entire article, you will notice that this only works on
machines that are running NT 3.51 or 4.0 SP1.  Anything prior to SP1 is not
at risk.

Why did these guys bother publishing this??

>This was cut from the March 10th, 1999 Windows NT Magazine Security Update
>Buletin:
>
>* NT SCREENSAVER VULNERABILITY
>
>Prasad Dabak, of Cybermedia Software Private Limited, has discovered a
>
>vulnerability in the Windows NT screensaver where a user could possibly
><http://www.ntsecurity.net/scripts/load.asp?iD=/security/screensaver1.htm>

-----------------------------------------------------------------------------

Date: Wed, 10 Mar 1999 22:06:52 -0500
From: Russ <Russ.Cooper@RC.ON.CA>
To: NTBUGTRAQ@LISTSERV.NTBUGTRAQ.COM
Subject: Re: NT SCREENSAVER VULNERABILITY

>And if you read the entire article, you will notice that this only
works
>on machines that are running NT 3.51 or 4.0 SP1.  Anything prior to SP1
>is not at risk.

I assume you mean anything after SP1 is not at risk.

I've been told that the exposure discovered by Cybermedia does, in fact,
exist in versions after NT 4.0 SP1, but is "far more complex" to
execute. Ergo, they haven't been able to construct a working
demonstration exploit on systems running SP2 or greater.

As such, it may only be a matter of time until these "complexities" are
resolved and an exploit released. I'm further told that Microsoft will
be releasing a fix/patch for this.

Its worth noting that Cybermedia said the exploit was demonstrable in
Win2000 B2, implying the fault continues to exist.

When I first received notice of this from Mark Edwards of NTShop, like
you, I discounted the worthiness of a report to NTBugtraq. However, if
what Mark says is true (and no reason to doubt it), this "heads-up" is
worth it.

Let us not, however, get into any discussions about Screen Saver
Security in general, that's not the issue here. What's at issue is the
way Winlogon.exe handles the situation created by the demonstration
program (which happens to be a screen saver).

>why did these guys bother publishing this??

If nothing else, consider it useful FYI. If Microsoft confirms the
problem continues to exist in SP2/SP3/SP4, consider this very useful
information.

Cheers,
Russ - NTBugtraq moderator

-----------------------------------------------------------------------------

Date: Sat, 13 Mar 1999 00:29:40 -0800
From: aleph1@UNDERGROUND.ORG
To: BUGTRAQ@netspace.org
Subject: Microsoft Security Bulletin (MS99-008)

The following is a Security  Bulletin from the Microsoft Product Security
Notification Service.

Please do not  reply to this message,  as it was sent  from an unattended
mailbox.
                    ********************************

Microsoft Security Bulletin (MS99-008)
--------------------------------------

Patch Available for Windows NT "Screen Saver" Vulnerability

Originally Posted: March 12, 1999

Summary
=======
Microsoft has learned of a vulnerability affecting all versions of
Microsoft(r) Windows NT(r) operating system, which could allow a user to
gain administrative privileges on a computer. In most common usage
scenarios, this vulnerability presents itself on workstations, terminal
servers, and other systems that allow non-administrative users to
interactively log on. Less-common configurations could also be affected, and
are discussed below.

A fully supported patch is available to eliminate the vulnerability, and
Microsoft recommends that affected customers download and install it, if
appropriate.

Issue
=====
Windows NT provides a screen saver feature, in which a user-selected screen
saver program is run when the machine has been idle for a specified length
of time. Windows NT initially launches a screen saver in the local system
context, then immediately changes its security context to match that of the
user. However, Windows NT does not check whether this context change was
successfully made. This is the underlying problem in this vulnerability. If
the context change can be made to fail, the screen saver will remain running
in a highly-privileged state. The risk is that a malicious user could
develop a screen saver program that, for example, uses the elevated
privileges to add the author to the Administrators group.

It is important to understand that the user must able to run exploitation
code on a machine in order to elevate their privileges. There are two types
of machines at risk:
 - Machines that allow non-administrative users to
   interactively log on. Workstation and terminal servers
   typically do allow this, but, per standard security practices,
   most other machines only allow administrators to interactively
   log on.
 - Machines that allow remote users to submit arbitrary programs
   for execution. Servers such as domain controllers, line of
   business servers, application servers, print and file servers
   and the like typically do not accept arbitrary programs for
   execution.

It also is important to note that the scope of the privilege elevation is
highly dependent on the specific machine on which the exploitation code is
run. For example, a user who exploited this vulnerability on a workstation
could join the local Administrators group, but could not directly exploit
this vulnerability to become a domain administrator. However, a user who
exploited this vulnerability on a domain controller would be able to become
a domain Administrator, because the domain SAM is shared among all domain
controllers.

While there are no reports of customers being adversely affected by this
vulnerability, Microsoft is proactively providing a patch to allow customers
to take appropriate action to protect themselves against it.

Affected Software Versions
==========================
Microsoft Windows NT Workstation 4.0
Microsoft Windows NT Server 4.0
Microsoft Windows NT Server 4.0, Enterprise Edition
Microsoft Windows NT Server 4.0, Terminal Server Edition

What Microsoft is Doing
=======================
Microsoft has released patches that fix the problem identified. The patches
are available for download from the sites listed below in What Customers
Should Do.

Microsoft also has sent this security bulletin to customers
subscribing to the Microsoft Product Security Notification Service.
See http://www.microsoft.com/security/services/bulletin.asp
for more information about this free customer service.

Microsoft has published the following Knowledge Base (KB) article on this
issue:
 - Microsoft Knowledge Base (KB) article Q221991,
   Screen Saver Vulnerability Lets User Privileges be Elevated.
   http://support.microsoft.com/support/kb/articles/q221/9/91.asp
   (Note: It might take 24 hours from the original posting of this
   bulletin for the KB article to be visible in the Web-based
   Knowledge Base.)

What Customers Should Do
========================
Microsoft recommends that customers evaluate the degree of risk that this
vulnerability poses to their systems and determine whether to download and
install the patch. The patch can be found at:
 - X86-based Windows NT Workstation and Server 4.0 (including
   Enterprise Edition):
   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
   usa/NT40/hotfixes-postSP4/ScrnSav-fix/Scrnsavi.exe
 - X86-based Windows NT Server 4.0, Terminal Server Edition:
   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
   usa/NT40TSE/hotfixes-postSP3/ScrnSav-fix/Scrnsavi.exe
 - Alpha-based Windows NT Workstation and Server 4.0 (including
   Enterprise Edition):
   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
   usa/NT40/hotfixes-postSP4/ScrnSav-fix/Scrnsava.exe
 - Alpha-based Windows NT Server 4.0, Terminal Server Edition:
   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
   usa/NT40TSE/hotfixes-postSP3/ScrnSav-fix/Scrnsava.exe

(Note: The above URLs have been wrapped for readability)

(Note: Patches for Windows NT Server 4.0, Terminal Server Edition, will not
be available immediately upon release of this bulletin, but will be released
shortly after.)

More Information
================
Please see the following references for more information related to this
issue.
 - Microsoft Security Bulletin MS99-008,
   Patch Available for Windows NT 'Screen Saver' Vulnerability
   (the Web-posted version of this bulletin),
   http://www.microsoft.com/security/bulletins/ms99-008.asp.
 - Microsoft Knowledge Base (KB) article Q221991,
   Screen Saver Vulnerability Lets User Privileges be Elevated,
   http://support.microsoft.com/support/kb/articles/q221/9/91.asp
   (Note: It might take 24 hours from the original posting of this
   bulletin for the KB article to be visible in the Web-based
   Knowledge Base.)

Obtaining Support on this Issue
===============================
If you require technical assistance with this issue, please contact
Microsoft Technical Support. For information on contacting Microsoft
Technical Support, please see
http://support.microsoft.com/support/contact/default.asp.

Revisions
=========
 - March 12, 1999: Bulletin Created


For additional security-related information about Microsoft products,
please visit http://www.microsoft.com/security


-----------------------------------------------------------------

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LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE
FOREGOING LIMITATION MAY NOT APPLY.

(c) 1999 Microsoft Corporation. All rights reserved. Terms of Use.

   *******************************************************************
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-----------------------------------------------------------------------------

Date: Sat, 13 Mar 1999 02:38:08 -0500
From: Russ <Russ.Cooper@RC.ON.CA>
To: NTBUGTRAQ@LISTSERV.NTBUGTRAQ.COM
Subject: Alert: Microsoft Security Bulletin (MS99-008) - NT Screensaver Vulnerability

Microsoft have released a Security Bulletin
<http://www.microsoft.com/security/bulletins/ms99-008.asp> which covers
an issue raised by Cybermedia Software Private Limited in their March
10th, 1999 announcement
<http://www.cybermedia.co.in/NT_Security/SS_vulnerability.htm>.

Microsoft's bulletin is more forth-coming than usual in pointing out the
potential for exploit, something I'm sure we all welcome.

Although they do not explicitly state it, the fact that a ScrnSav-fix
directory has been added under the Post-SP3 and Post-SP4 directories of
NT 4.0 and NT 4.0 Terminal Server (intel and alpha), I guess its safe to
say that the exploit could've been made to work on releases beyond SP1
(as was originally reported). Small savings are gained from Cybermedia's
inability to create a demonstration exploit for SP3 or SP4 I guess (not
that someone won't do one in the future).

Anyone thinking of implementing this fix should seriously look at the
security of user profiles. Screen Savers represent all sorts of other
potential issues, and if properly controlled prior to the announcement
of this exploit, the exploit probably didn't represent much of a direct
threat.

That said, the lack of verification of a security context change in a
core component of NT is, as MS put it in their Bulletin, "the underlying
problem". The daunting size of NT, in terms of secure programming
practice verification, is truly showing if this functionality has not
been verified throughout all NT applications. Stop and count the number
of processes you can think of that do security context switching...;-[

Microsoft have prepared a KB article
<http://support.microsoft.com/support/kb/articles/q221/9/91.asp>
describing the vulnerability. This article was not available when I
checked.

Finally, while not directly related to this issue, I had a conversation
with individuals representing Cybermedia Software Private Limited. In
particular I discussed with them the issue of releasing exploits to the
"media" without receiving any response from the vendor (or waiting any
amount of time for such a response).

Its certainly their right, as it is anyone's, to do this. However, in
the spirit of White Hat Infosec, I sought to encourage them to, um, do
things differently in the future.

Their message to me was that they would "definitely notify Microsoft
ahead of time and adopt that as a policy". I offered them, as I hope you
all know I offer you, NTBugtraq's full support and capabilities for
getting the right people involved when a discovery needs to be
addressed.

Cheers,
Russ - NTBugtraq moderator