nt.screensaver.token.txt

`Date: Tue, 9 Mar 1999 12:57:42 -0800  
From: Aleph One <[email protected]>  
To: [email protected]  
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 <[email protected]>  
To: [email protected]  
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 <[email protected]>  
To: [email protected]  
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: [email protected]  
To: [email protected]  
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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-----------------------------------------------------------------------------  
  
Date: Sat, 13 Mar 1999 02:38:08 -0500  
From: Russ <[email protected]>  
To: [email protected]  
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  
  
`