Restricting Changes to Base System Objects

ID: Q218473

The information in this article applies to:
  • Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3, 4.0 SP4
  • Microsoft Windows NT Server versions 3.5, 3.51, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3, 4.0 SP4
  • Microsoft Windows NT Server, Enterprise Edition versions 4.0, 4.0 SP4
  • Microsoft Windows NT Server version 4.0, Terminal Server Edition

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SYMPTOMS

If a non-administrative level user logs on interactively to your computer, it may be possible for the user to gain local administrative privileges. A user may load a non-Microsoft dynamic-link library (DLL) into memory having the same name as a core operating system DLL and then programmatically change the entry in the KnownDLLs list to point to the malicious copy. Programs then requesting the system DLL will instead be directed to the malicious copy. When called by a program with sufficiently high security privileges, it may be possible to bypass typical security related activities such as adding a user to the Local Administrators group. A remote user may also submit an arbitrary program to run on your computer to exploit this vulnerability.


CAUSE

In Windows NT, core operating system DLLs are kept in virtual memory and shared between programs. This prevents redundant copies of the DLL in memory, and improves memory usage and system performance. When a program calls a function provided by one of these DLLs, the operating system refers to 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/write access to the KnownDLLs list.

This vulnerability is only local to the computer where a malicious user is logged on interactively. For example, a workstation user could exploit this vulnerability to become a local administrator, but cannot use it directly to become a domain administrator. This restricts the impact of this issue as non-administrative users are not granted the ability to interactively log on to servers (following recommended Microsoft security practices).


RESOLUTION

NOTE: You should apply this fix to all systems that allow non-privileged users to log in to them.

Windows NT 4.0

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:
For information on obtaining the individual software update, contact Microsoft Product Support Services. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
http://www.microsoft.com/support/supportnet/overview/overview.asp
This hotfix has been posted to the following Internet location as Smssfixi.exe (x86) and Smssfixa.exe (Alpha):
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/hotfixes-postSP4/Smss-fix/

Windows NT Server 4.0, Terminal Server Edition

To resolve this problem, obtain the latest service pack for Windows NT Server 4.0, Terminal Server Edition. For additional information, please see the following article in the Microsoft Knowledge Base:
Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack
This hotfix has been posted to the following Internet location as Smssfixi.exe (x86) and Smssfixa.exe (Alpha):
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40TSE/hotfixes-postSP3/Smss-fix/

Windows NT 3.51

A supported fix that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem.

To resolve this problem, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
http://www.microsoft.com/support/supportnet/overview/overview.asp
The English version of this fix should have the following file attributes or later:

   Date      Time                 Size    File name     Platform
   -------------------------------------------------------------
   03/24/99  09:56p                37,328 Smss.exe      (x86)
   03/24/99  09:59p                54,544 Smss.exe      (Alpha)


WORKAROUND

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

To work around this issue, enable stronger protection on system base objects such as the KnownDLLs list by doing the following:

  1. Click Start, click Run, type regedt32, and then click OK.


  2. On the Windows menu, click HKEY_LOCAL_ MACHINE on Local Machine.


  3. Double-click the \System\CurrentControlSet\Control\Session Manager folder.


  4. On the Edit menu, click Add Value, type ProtectionMode, click REG_DWORD in the Data Type box, and then click OK.


  5. Type 1 in the Data box, and then click OK.


  6. Quit Registry Editor, and then restart the computer.



STATUS

Microsoft has confirmed this problem could result in some degree of security vulnerability in the Microsoft products listed at the beginning of this article.

This problem was first corrected in Windows NT Server 4.0, Terminal Server Edition Service Pack 4. This problem was first corrected in Windows NT version 4.0 Service Pack 5.


MORE INFORMATION

For additional security related information about Microsoft products, please visit the Web site at:

http://www.microsoft.com/security/

Additional query words:

Keywords : kbbug4.00 kbfix4.00 nt4sp5fix TSESP4Fix
Version : winnt:3.5,3.51,4.0,4.0 SP1,4.0 SP2,4.0 SP3,4.0 SP4
Platform : winnt
Issue type : kbbug


Last Reviewed: February 1, 2000
© 2000 Microsoft Corporation. All rights reserved. Terms of Use.