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Microsoft Security Bulletin (MS99-008)
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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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