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Microsoft Security Bulletin (MS99-006)
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Fix Available for Windows NT "KnownDLLs List" Vulnerability
Originally Posted: February 19, 1999
Summary
=======
Microsoft has learned of a vulnerability in the Microsoft(r) Windows NT(r)
operating system that could allow a user to gain administrative privileges
on the computer the user is interactively logged onto. This vulnerability
is primarily a concern for workstations and other systems that allow
non-administrative users to interactively log on.
The privilege elevation can be prevented by setting the value of a specific
registry key. The registry setting is described below and also discussed in
multiple Windows NT security guidelines. In addition, Microsoft is
developing a hot fix that will change the default access control settings
on the relevant operating system data in order to eliminate the
vulnerability altogether. When this hot fix is available, an updated
version of this bulletin will be issued to provide more information.
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 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 Local Administrators group.
It is important to understand that this vulnerability is local to the
machine that the malicious user is interactively logged onto. For example,
a workstation user could exploit this vulnerability to become a local
administrator, but could not use it directly to become a domain
administrator. This fact restricts the impact of the vulnerability mainly to
workstations because, if recommended security practices are observed,
non-administrative users will not be granted the ability to interactively
log onto servers. Even on workstations, it is worth pointing out that most
workstation users already are local administrators and have full control
over the machine.
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 registry change detailed
below works by restricting users' ability to change system base objects,
including the KnownDLLs list. A hot fix will be provided shortly that
changes the default permissions on the KnownDLLs list to read-only.
Affected Software Versions
==========================
- Microsoft Windows NT 3.5, 3.51, and 4.0, Standard and Enterprise Editions
What Microsoft is Doing
=======================
Microsoft has provided an immediate workaround in the form of a registry
entry change, as discussed below in "What Customers Should Do". Microsoft
is developing a hot fix that will change the default permissions on the
KnownDLLs list, and will issue an update to this bulletin when it is
available.
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
========================
To enable stronger protection on system base objects such as the KnownDLLs
list, add the following value to the registry key
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SessionManager:
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 (URL has been word-wrapped for
readability)
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 would like to acknowledge L0pht Heavy Industries
(http://www.l0pht.com) for discovering this vulnerability.
Revisions
=========
- February 19, 1999: Bulletin Created
For additional security-related information about Microsoft
products, please visit http://www.microsoft.com/security
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