Microsoft Security Bulletin MS99-006 - Critical
Fix Available for Windows NT "KnownDLLs List" Vulnerability
Published: February 19, 1999 | Updated: March 10, 2003
Patch Availability Information Updated: March 10, 2003
Originally Posted: February 19, 1999
Updated: March 5, 1999
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® Windows NT® 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.
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
Vulnerability Identifier: CVE-1999-0376
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 The Microsoft Product Security Notification Service 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 218473, Restricting Changes to Base System Objects.
(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): https://www.microsoft.com/technet/security/patchavailability.mspx
- X86-based Windows NT Server 4.0, Terminal Server Edition: https://www.microsoft.com/technet/security/patchavailability.mspx
- Alpha-based Windows NT Workstation and Server 4.0 (including Enterprise Edition and Terminal Server Edition): https://www.microsoft.com/technet/security/patchavailability.mspx
- Alpha-based Windows NT Server 4.0, Terminal Server Edition: https://www.microsoft.com/technet/security/patchavailability.mspx
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.
Key: HKEY_LOCAL_MACHINE \System \CurrentControlSet \Control \Session Manager
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.
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), https://www.microsoft.com/technet/security/bulletin/ms99-006.mspx.
- Microsoft Knowledge Base (KB) article 218473, Restricting Changes to Base System Objects.
- Microsoft White Paper, Securing Windows NT Installation, available at https://www.microsoft.com/technet/security/default.mspx https://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 https:.
Microsoft would like to acknowledge l0pht Heavy Industries (https://www.lopht.com) for discovering this vulnerability.
- February 19, 1999: Bulletin Created
- March 5, 1999: Bulletin Updated
- V2.0 (March 10, 2003): Introduced versioning and updated patch availability information
For additional security-related information about Microsoft products, please visit https://www.microsoft.com/technet/security
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