Microsoft Security Bulletin MS04-020 Vulnerability in POSIX Could Allow Code Execution (841872)

Microsoft Security Bulletin MS04-020
Vulnerability in POSIX Could Allow Code Execution (841872)

Issued: July 13, 2004
Version: 1.0

Summary
Who should read this document: Customers who use Microsoft® Windows® 2000 or Windows NT 4.0

Impact of Vulnerability: Local Elevation of Privilege

Maximum Severity Rating: Important

Recommendation: Customers should install the update at the earliest opportunity.

Security Update Replacement: None

Caveats: None

Tested Software and Security Update Download Locations:

Affected Software:

• Microsoft Windows NT® Workstation 4.0 Service Pack 6a – Download the update

• Microsoft Windows NT Server 4.0 Service Pack 6a – Download the update

• Microsoft Windows NT Server 4.0 Terminal Server Edition Service Pack 6 – Download the update

• Microsoft Windows 2000 Service Pack 2, Microsoft Windows 2000 Service Pack 3, Microsoft Windows 2000 Service Pack 4 – Download the update

Non-Affected Software:

• Microsoft Windows XP and Microsoft Windows XP Service Pack 1

• Microsoft Windows XP 64-Bit Edition Service Pack 1

• Microsoft Windows XP 64-Bit Edition Version 2003

• Microsoft Windows Server™ 2003

• Microsoft Windows Server 2003 64-Bit Edition

• Microsoft Windows 98, Microsoft Windows 98 Second Edition (SE), and Microsoft Windows Millennium Edition (Me)

The software in this list has been tested to determine if the versions are affected. Other versions either no longer include security update support or may not be affected. To determine the support lifecycle for your product and version, visit the following Microsoft Support Lifecycle Web site.

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General Information
Executive Summary

Executive Summary:

This update resolves a newly-discovered, privately reported vulnerability. A privilege elevation vulnerability exists in the POSIX operating system component (subsystem). The vulnerability is documented in the Vulnerability Details section of this bulletin.

An attacker who successfully exploited this vulnerability could take complete control of an affected system, including installing programs; viewing, changing, or deleting data; or creating new accounts that have full privileges.

We recommend that customers install the update at the earliest opportunity.

Severity Ratings and Vulnerability Identifiers:

Vulnerability Identifiers Impact of Vulnerability Windows NT 4.0 Windows 2000
POSIX Vulnerability - CAN-2004-0210
Privilege Elevation
Important
Important

This assessment is based on the types of systems that are affected by the vulnerability, their typical deployment patterns, and the effect that exploiting the vulnerability would have on them.

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Frequently asked questions (FAQ) related to this security update

I'm still using Microsoft Windows NT 4.0 Workstation Service Pack 6a or Windows 2000 Service Pack 2, but extended security update support ended on June 30, 2004. However, this bulletin has a security update for these operating system versions. Why is that?
Windows NT 4.0 Workstation Service Pack 6a and Windows 2000 Service Pack 2 have reached the end of their life cycles as previously documented, and Microsoft extended this support to June 30, 2004. However, the end-of-life for the extended support period occurred very recently. In this case, the majority of the steps that are required to address this vulnerability were completed before June 30, 2004. Therefore, we have decided to release security updates for these operating system versions as part of this security bulletin. We do not anticipate doing this for future vulnerabilities affecting these operating system versions, but we reserve the right to produce updates and to make these updates available when necessary.

It should be a priority for customers who have these operating system versions to migrate to supported versions to prevent potential exposure to future vulnerabilities. For more information about the Windows Product Life Cycle, visit the following Microsoft Support Lifecycle Web site. For more information about the extended security update support period for these operating system versions, visit the following Microsoft Product Support Services Web site.

Customers who require additional support for Windows NT Workstation 4.0 SP6a must contact their Microsoft account team representative, their Technical Account Manager, or the appropriate Microsoft partner representative for custom support options. Customers without an Alliance, Premier, or Authorized Contract can contact their local Microsoft sales office. For contact information, visit the Microsoft Worldwide Information Web site, select the country, and then click Go to see a list of phone numbers. When you call, ask to speak with the local Premier Support sales manager.

For more information, see the Windows Operating System FAQ.

Can I use the Microsoft Baseline Security Analyzer (MBSA) to determine if this update is required?
Yes. MBSA will determine if this update is required. For more information about MBSA, visit the MBSA Web site.

Note After April 20, 2004, the Mssecure.xml file that is used by MBSA 1.1.1 and earlier versions is no longer being updated with new security bulletin data. Therefore, scans that are performed after that date with MBSA 1.1.1 or earlier will be incomplete. All users should upgrade to MBSA 1.2 because it provides more accurate security update detection and supports additional products. Users can download MBSA 1.2 from the MBSA Web site. For more information about MBSA support, visit the following Microsoft Baseline Security Analyzer 1.2 Q&A Web site.

Can I use Systems Management Server (SMS) to determine if this update is required?
Yes. SMS can help detect and deploy this security update. For information about SMS, visit the SMS Web site.

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Vulnerability Details

POSIX Vulnerability - CAN-2004-0210

A privilege elevation vulnerability exists in the POSIX subsystem. This vulnerability could allow a logged on user to take complete control of the system.

Mitigating Factors for POSIX Vulnerability - CAN-2004-0210:

• An attacker must have valid logon credentials and be able to logon locally to exploit this vulnerability. The vulnerability could not be exploited remotely or by anonymous users.

• Windows XP and Windows Server 2003 are not affected by this vulnerability.

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Workarounds for POSIX Vulnerability - CAN-2004-0210:

Microsoft has tested the following workarounds. While these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified below.

• Disable the POSIX subsystem through the registry

This workaround is fully documented in Microsoft Knowledge Base Article 101270. This article is summarized in the following paragraphs.

The following steps demonstrate how to disable the POSIX subsystem.

Note 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 view the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe.

Note We recommend backing up the registry before you edit it.

Click Start, click Run, type "regedt32" (without the quotation marks), and then click OK.

In Registry Editor, locate the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Subsystems\Posix

Click the POSIX data value, click Edit, and then click Delete.

Click OK to confirm the delete, and then restart the system.

Note To enable the POSIX subsystem, re-create the registry key. The name if the registry key is Posix, the type of registry key is REG_EXPAND_SZ, and the registry key value is %SystemRoot%\system32\psxss.exe. After you have done this, restart the system.

Impact of Workaround: POSIX programs are disabled until the POSIX subsystem is enabled.

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FAQ for POSIX Vulnerability - CAN-2004-0210:

What is the scope of the vulnerability?
This is a privilege elevation vulnerability. An attacker who successfully exploited this vulnerability could take complete control of an affected system, including installing programs; viewing, changing, or deleting data; or creating new accounts that have full privileges.

What causes the vulnerability?
An unchecked buffer in the POSIX subsystem.

What is the POSIX subsystem?
You can run applications that are created for the Portable Operating System Interface for UNIX (POSIX) standard under Windows NT 4.0 and Windows 2000. The operating systems provide support for nonnative applications by emulating the environments in which they are designed to be processed. This support is provided through environment subsystems. Except for the Microsoft Win32 subsystem, which is the native environment of Windows, each environment is optional and is used only when a client application requires its services. For more information about POSIX support, visit the following MSDN Library Web Site.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could take complete control of the affected system.

Who could exploit the vulnerability?
To exploit the vulnerability, an attacker must be able to log on locally to a system that has the POSIX subsystem enabled.

How could an attacker exploit the vulnerability?
To exploit this vulnerability, an attacker would first have to log on to the system. An attacker could then run a specially-designed program that could attempt to exploit the vulnerability, and thereby gain complete control over the affected system.

An attacker could also access the affected component through another vector. For example, an attacker could use another program that passes parameters to the vulnerable component (locally or remotely).

What systems are primarily at risk from the vulnerability?
Windows NT 4.0 and Windows 2000 systems are at risk from this vulnerability. Windows XP and Windows Server 2003 do not contain the POSIX subsystem. For more information about the support of POSIX in Windows XP and in Windows Server 2003, see Microsoft Knowledge Base Article 308259.

Workstations and terminal servers are primarily at risk. Servers are only at risk if users who do not have sufficient administrative credentials are given the ability to log on to servers and to run programs. However, best practices strongly discourage allowing this.

Could the vulnerability be exploited over the Internet?
No. An attacker must be able to log on to the specific system that is targeted for attack. An attacker cannot load and run a program remotely by exploiting this vulnerability.

What does the update do?
The update removes the vulnerability by modifying the way that the POSIX subsystem validates the length of a message before it passes the message to the allocated buffer.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
No. Microsoft received information about this vulnerability through responsible disclosure. Microsoft had not received any information indicating that this vulnerability had been publicly disclosed when this security bulletin was originally issued.

When this security bulletin was issued, had Microsoft received any reports that this vulnerability was being exploited?
No. Microsoft had not received any information indicating that this vulnerability had been publicly used to attack customers and had not seen any examples of proof of concept code published when this security bulletin was originally issued.

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Security Update Information

Installation Platforms and Prerequisites:

For information about the specific security update for your platform, click the appropriate link:

Windows 2000 (all versions)

Prerequisites
For Windows 2000, this security update requires Service Pack 2 (SP2), Service Pack 3 (SP3), or Service Pack 4 (SP4).

The software that is listed has been tested to determine if the versions are affected. Other versions either no longer include security update support or may not be affected. To determine the support lifecycle for your product and version, visit the Microsoft Support Lifecycle Web site.

For more information about how to obtain the latest service pack, see Microsoft Knowledge Base Article 260910.

Inclusion in Future Service Packs:
The update for this issue will be included in Windows 2000 Service Pack 5.

Installation Information

This security update supports the following setup switches:

  /help                 Displays the command line options

Setup Modes

  /quiet                Quiet mode (no user interaction or display)

  /passive            Unattended mode (progress bar only)

  /uninstall          Uninstalls the package

Restart Options

  /norestart          Do not restart when installation is complete

  /forcerestart      Restart after installation

Special Options

  /l                        Lists installed Windows hotfixes or update packages

  /o                       Overwrite OEM files without prompting

  /n                       Do not backup files needed for uninstall

  /f                        Force other programs to close when the computer shuts down

  /extract             Extracts files without starting setup 

Note You can combine these switches into one command. For backward compatibility, the security update also supports the setup switches that the previous version of the setup utility uses. For more information about the supported installation switches, see Microsoft Knowledge Base Article 262841.

Deployment Information

To install the security update without any user intervention, use the following command at a command prompt for Windows 2000 Service Pack 2, Windows 2000 Service Pack 3, and Windows 2000 Service Pack 4:

Windows2000-kb841872-x86-enu /passive /quiet

To install the security update without forcing the system to restart, use the following command at a command prompt for Windows 2000 Service Pack 2, Windows 2000 Service Pack 3, and Windows 2000 Service Pack 4:

Windows2000-kb841872-x86-enu /norestart

For more information about how to deploy this security update with Software Update Services, visit the Software Update Services Web site.

Restart Requirement

In some cases, this update does not require a restart. The installer stops the required services, applies the update, and then restarts the services. However, if the required services cannot be stopped for any reason or if required files are in use, this update will require a restart. If this occurs, a message appears that advises you to restart.

Removal Information

To remove this security update, use the Add or Remove Programs tool in Control Panel.

System administrators can also use the Spuninst.exe utility to remove this security update. The Spuninst.exe utility is located in the %Windir%\$NTUninstallKB841872$\Spuninst folder. The Spuninst.exe utility supports the following setup switches:

/?: Show the list of installation switches.

/u: Use unattended mode.

/f: Force other programs to quit when the computer shuts down.

/z: Do not restart when the installation is complete.

/q: Use Quiet mode (no user interaction).

File Information

The English version of this update has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.

Note Date, time, file name, or size information could change during installation. Refer to the Verifying Update Installation section for details on verifying an installation.

Windows 2000 Service Pack 2, Windows 2000 Service Pack 3, and Windows 2000 Service Pack 4:

Date Time Version Size File name

16-May-2004 19:32 5.0.2195.6929 90,384 Psxss.exe

Verifying Update Installation

• Microsoft Baseline Security Analyzer

To verify that a security update is installed on an affected system, you may be able to use the Microsoft Baseline Security Analyzer (MBSA) tool. This tool allows administrators to scan local and remote systems for missing security updates and for common security misconfigurations. For more information about MBSA, visit the Microsoft Baseline Security Analyzer Web site.

• File Version Verification

Note Because there are several versions of Microsoft Windows, the following steps may be different on your computer. If they are, see your product documentation to complete these steps.

Click Start, and then click Search.

In the Search Results pane, click All files and folders under Search Companion.

In the All or part of the file name box, type a file name from the appropriate file information table, and then click Search.

In the list of files, right-click a file name from the appropriate file information table, and then click Properties.

Note Depending on the version of the operating system or programs installed, some of the files that are listed in the file information table may not be installed.

On the Version tab, determine the version of the file that is installed on your computer by comparing it to the version that is documented in the appropriate file information table.

Note Attributes other than file version may change during installation. Comparing other file attributes to the information in the file information table is not a supported method of verifying the update installation. Also, in certain cases, files may be renamed during installation. If the file or version information is not present, use one of the other available methods to verify update installation.

• Registry Key Verification

You may also be able to verify the files that this security update has installed by reviewing the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows 2000\SP5\KB841872\Filelist

Note This registry key may not contain a complete list of installed files. Also, this registry key may not be created correctly when an administrator or an OEM integrates or slipstreams the 841872 security update into the Windows installation source files.

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Windows NT 4.0 (all versions)

Prerequisites
This security update requires Windows NT Workstation 4.0 Service Pack 6a (SP6a), Windows NT Server 4.0 Service Pack 6a (SP6a), or Windows NT Server 4.0 Terminal Server Edition Service Pack 6 (SP6).

The software that is listed has been tested to determine if the versions are affected. Other versions either no longer include security update support or may not be affected. To determine the support lifecycle for your product and version, visit the following Microsoft Support Lifecycle Web site.

For more information about obtaining the latest service pack, see Microsoft Knowledge Base Article 152734.

Installation Information

This security update supports the following setup switches:

/y: Perform removal (only with /m or /q )

/f: Force programs to quit during the shutdown process

/n: Do not create an Uninstall folder

/z: Do not restart when the update completes

/q: Use Quiet or Unattended mode with no user interface (this switch is a superset of /m )

/m: Use Unattended mode with a user interface

/l: List the installed hotfixes

/x: Extract the files without running Setup

Note You can combine these switches into one command. For more information about the supported installation switches, see Microsoft Knowledge Base Article 262841.

Deployment Information

To install the security update without any user intervention, use the following command at a command prompt for Windows NT Server 4.0:

Windowsnt4server-kb841872-x86-enu /q

For Windows NT Server 4.0 Terminal Server Edition:

Windowsnt4terminalserver-kb841872-x86-enu /q

For Windows NT Workstation 4.0:

Windowsnt4workstation-kb841872-x86-enu /q

To install the security update without forcing the system to restart, use the following command at a command prompt for Windows NT Server 4.0:

Windowsnt4server-kb841872-x86-enu /z

For Windows NT Server 4.0 Terminal Server Edition:

Windowsnt4terminalserver-kb841872-x86-enu /z

For Windows NT Workstation 4.0:

Windowsnt4workstation-kb841872-x86-enu /z

For more information about how to deploy this security update with Software Update Services, visit the Software Update Services Web site.

Restart Requirement

In some cases, this update does not require a restart. The installer stops the needed services, applies the update, and then restarts the services. However, if the required services cannot be stopped for any reason or if required files are in use, this update will require a restart. If this occurs, a message appears that advises you to restart.

Removal Information

To remove this security update, use the Add/Remove Programs tool in Control Panel.

System administrators can also use the Hotfix.exe utility to remove this security update. The Hotfix.exe utility is located in the %Windir%\$NTUninstallKB841872$ folder. The Hotfix.exe utility supports the following setup switches:

/y: Perform removal (only with the /m or /q switch)

/f: Force programs to quit during the shutdown process

/n: Do not create an Uninstall folder

/z: Do not restart when the installation is complete

/q: Use Quiet or Unattended mode with no user interface (this switch is a superset of the /m switch)

/m: Use Unattended mode with a user interface

/l: List the installed hotfixes

File Information

The English version of this update has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.

Note Date, time, filename, or size information could change during installation. Refer to the Verifying Update Installation section for details on verifying an installation.

Windows NT Workstation 4.0 and Windows NT Server 4.0:

Date Time Version Size File name

20-May-2004 15:04 4.0.1381.7269 93,968 Psxss.exe

Windows NT Server 4.0 Terminal Server Edition:

Date Time Version Size File name

21-May-2004 13:31 4.0.1381.33567 94,480 Psxss.exe

Verifying Update Installation

• Microsoft Baseline Security Analyzer

To verify that a security update is installed on an affected system, you may be able to use the Microsoft Baseline Security Analyzer (MBSA) tool. This tool allows administrators to scan local and remote systems for missing security updates and for common security misconfigurations. For more information about MBSA, visit the Microsoft Baseline Security Analyzer Web site.

• File Version Verification

Note Because there are several versions of Microsoft Windows, the following steps may be different on your computer. If they are, see your product documentation to complete these steps.

Click Start, and then click Search.

In the Search Results pane, click All files and folders under Search Companion.

In the All or part of the file name box, type a file name from the appropriate file information table, and then click Search.

In the list of files, right-click a file name from the appropriate file information table, and then click Properties.

Note Depending on the version of the operating system or programs installed, some of the files that are listed in the file information table may not be installed.

On the Version tab, determine the version of the file that is installed on your computer by comparing it to the version that is documented in the appropriate file information table.

Note Attributes other than file version may change during installation. Comparing other file attributes to the information in the file information table is not a supported method of verifying the update installation. Also, in certain cases, files may be renamed during installation. If the file or version information is not present, use one of the other available methods to verify update installation.

• Registry Key Verification

You may also be able to verify the files that this security update has installed by reviewing the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Hotfix\KB841872\File 1

Note This registry key may not contain a complete list of installed files. Also, this registry key may not be created correctly when an administrator or an OEM integrates or slipstreams the 841872 security update into the Windows installation source files.

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Acknowledgments

Microsoft thanks the following for working with us to help protect customers:

• Rafal Wojtczuk working with McAfee for reporting the POSIX Vulnerability (CAN-2004-0210).

Obtaining Other Security Updates:

Updates for other security issues are available from the following locations:

• Security updates are available from the Microsoft Download Center: You can find them most easily by doing a keyword search for "security_patch".

• Updates for consumer platforms are available from the Windows Update Web site.

Support:

• Customers in the U.S. and Canada can receive technical support from Microsoft Product Support Services at 1-866-PCSAFETY. There is no charge for support calls that are associated with security updates.

• International customers can receive support from their local Microsoft subsidiaries. There is no charge for support that is associated with security updates. For more information about how to contact Microsoft for support issues, visit the International Support Web site.

Security Resources:

• The Microsoft TechNet Security Web site provides additional information about security in Microsoft products.

• Microsoft Software Update Services

• Microsoft Baseline Security Analyzer (MBSA)

• Windows Update

• Windows Update Catalog: For more information about the Windows Update Catalog, see Microsoft Knowledge Base Article 323166.

• Office Update

Software Update Services:

By using Microsoft Software Update Services (SUS), administrators can quickly and reliably deploy the latest critical updates and security updates to Windows 2000 and Windows Server 2003-based servers, and to desktop systems that are running Windows 2000 Professional or Windows XP Professional.

For more information about how to deploy this security update with Software Update Services, visit the Software Update Services Web site.

Systems Management Server:

Microsoft Systems Management Server (SMS) delivers a highly-configurable enterprise solution for managing updates. By using SMS, administrators can identify Windows-based systems that require security updates and to perform controlled deployment of these updates throughout the enterprise with minimal disruption to end users. For more information about how administrators can use SMS 2003 to deploy security updates, see the SMS 2003 Security Patch Management Web site. SMS 2.0 users can also use Software Updates Service Feature Pack to help deploy security updates. For information about SMS, visit the SMS Web site.

Note SMS uses the Microsoft Baseline Security Analyzer and the Microsoft Office Detection Tool to provide broad support for security bulletin update detection and deployment. Some software updates may not be detected by these tools. Administrators can use the inventory capabilities of the SMS in these cases to target updates to specific systems. For more information about this procedure, see the following Web site. Some security updates require administrative rights following a restart of the system. Administrators can use the Elevated Rights Deployment Tool (available in the SMS 2003 Administration Feature Pack and in the SMS 2.0 Administration Feature Pack) to install these updates.

Disclaimer:

The information provided in the Microsoft Knowledge Base is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.

Revisions:

• V1.0 (July 13, 2004): Bulletin published