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Update to Microsoft Security Bulletin (MS99-006)
------------------------------------------------
Fix Available for Windows NT "KnownDLLs List" Vulnerability
Originally Posted: February 19, 1999
Updated: March 5, 1999
Summary
=======
This is an update to Microsoft Security MS99-006, which was originally
issued on February 19, 1999. Microsoft is issuing this updated bulletin to
inform customers of the availability of a patch, and to update the list of
affected products.
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.
The privilege elevation can be prevented by applying a hot fix that changes
the default access control settings on the relevant operating system
object. The hot fix is available for downloading from the Microsoft FTP
site. Microsoft recommends that customers who previously made a registry
change as a temporary workaround revert to the original registry setting and
use the hot fix instead.
Issue
=====
In Windows NT, core operating system DLLs are kept in virtual memory and
shared between the programs running on the system. This is done to avoid
having redundant copies of the DLLs in memory, and improves memory usage
and system performance. When a program calls a function provided by one of
these DLLs, the operating system references a data structure called the
KnownDLLs list to determine the location of the DLL in virtual memory. The
Windows NT security architecture protects in-memory DLLs against
modification, but by default it allows all users to read from and write to
the KnownDLLs list. This is the root problem underlying the vulnerability.
A user can programmatically load into memory a malicious DLL that has the
same name as a system DLL, then change the entry in the KnownDLLs list to
point to the malicious copy. From that point forward, programs that request
the system DLL will instead be directed to the malicious copy. When called
by a program with sufficiently high privileges, it could take any desired
action, such as adding the malicious user 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 servers only allow
administrators to interactively log on. (Even on workstations, it's
worth noting that most workstation users already are administrators
on the local machine).
- 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
privilege elevation vulnerability, Microsoft is proactively providing
information to allow customers to prevent it. The hot fix changes the
default permissions on the KnownDLLs list to read-only, and is the
recommended corrective action for this vulnerability. The initial version of
this bulletin provided a workaround in the form of a registry change that
restricts users' ability to change system base objects, including the
KnownDLLs list. Although the registry change corrects the problem, it
encompasses a broader range of system behavior than the hot fix, and may not
be appropriate for all systems.
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 provided a patch that changes the default permissions on the
KnownDLLs list. Information on the patch is provided 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 Q218473,
Restricting Changes to Base System Objects,
http://support.microsoft.com/support/kb/articles/q218/4/73.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 highly recommends that customers evaluate the degree of risk that
this vulnerability poses to their systems and determine whether to download
and install the hot fix. The hot fix changes the default permissions on the
KnownDLLs list, and is the recommended means of eliminating the
vulnerability.
The hot fix 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/Smss-fix/Smssfixi.exe
- X86-based Windows NT Server 4.0, Terminal Server Edition:
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
usa/NT40TSE/hotfixes-postSP3/Smss-fix/Smssfixi.exe
- Alpha-based Windows NT Workstation and Server 4.0
(including Enterprise Edition and Terminal Server Edition):
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
usa/NT40/hotfixes-postSP4/Smss-fix/Smssfixa.exe
- Alpha-based Windows NT Server 4.0, Terminal Server Edition:
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/
usa/NT40TSE/hotfixes-postSP3/Smss-fix/Smssfixa.exe
(Note: the above URLs have been word-wrapped for readability)
Registry Change
===============
It is also possible to eliminate this vulnerability via a registry change
that enables stronger protection on system base objects such as the
KnownDLLs list. However, because this registry change affects all system
base objects, rather than just the KnownDLLs list, it may not be
appropriate for all systems. The recommended fix for this vulnerability is
via the hot fix detailed above in What Customers Should Do. Customers who
previously used this registry change as a temporary workaround may wish to
revert to their original setting and install the hot fix as a permanent
solution.
Registry Change:
- Key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Control\Session Manager
(note: the key name has been word-wrapped for readability)
- Name: ProtectionMode
- Type: REG_DWORD
- Value: 1
NOTE: Incorrectly changing the system registry can damage your system or
render it inoperable, and users undertake these changes at their own risk.
If you require assistance in making this change, see Obtaining Support on
this Issue below.
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-006,
Fix Available for Windows NT "KnownDLLs List" Vulnerability
(the Web-posted version of this bulletin),
http://www.microsoft.com/security/bulletins/ms99-006.asp.
- Microsoft Knowledge Base (KB) article Q218473,
Restricting Changes to Base System Objects,
http://support.microsoft.com/support/kb/articles/q218/4/73.asp.
- Microsoft White Paper, Securing Windows NT Installation, available
at http://www.microsoft.com/security/resources/whitepapers.asp and
http://www.microsoft.com/ntserver/security/exec/
overview/Secure_NTInstall.asp
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.
Acknowledgments
===============
Microsoft acknowledges L0pht Heavy Industries (http://www.l0pht.com)
for discovering this vulnerability.
Revisions
=========
- February 19, 1999: Bulletin Created
- March 5, 1999: Bulletin Updated
For additional security-related information about Microsoft products,
please visit http://www.microsoft.com/security
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