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Microsoft Security Bulletin MS04-042 - Important

Vulnerability in DHCP Could Allow Remote Code Execution and Denial of Service (885249)

Published: December 14, 2004

Version: 1.0

Issued: December 14, 2004
Version: 1.0

Summary

Who should read this document: Customers who use the Microsoft DHCP Server service.

Impact of Vulnerability:  Remote Code Execution

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:

Non-Affected Software:

  • Microsoft Windows 2000 Service Pack 3 and Microsoft Windows 2000 Service Pack 4
  • Microsoft Windows XP Service Pack 1 and Microsoft Windows XP Service Pack 2
  • 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.

General Information

Executive Summary:

This update resolves several newly-discovered, privately reported vulnerabilities. Each vulnerability is documented in this bulletin in its own Vulnerability Details section.

An attacker who successfully exploited the most severe of these vulnerabilities could take complete control of an affected system, including installing programs; viewing, changing, or deleting data; or creating new accounts that have full privileges. However, attempts to exploit these vulnerabilities would most likely result in a denial of service of the Dynamic Host Configuration Protocol (DHCP) Server service.

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

Severity Ratings and Vulnerability Identifiers:

Vulnerability IdentifiersImpact of VulnerabilityWindows NT 4.0
Logging Vulnerability - CAN-2004-0899Denial of ServiceModerate
DHCP Request Vulnerability - CAN-2004-0900Remote Code ExecutionImportant
Aggregate Severity of All Vulnerabilities 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.

Why does this update address several reported security vulnerabilities?
This update contains support for several vulnerabilities because the modifications that are required to address these issues are located in related files. Instead of having to install several updates that are almost the same, customers can install only this update.

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 1.2.1 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. The Security Update Inventory Tool is required for detecting Microsoft Windows and other affected Microsoft products. For more information about the limitations of the Security Update Inventory Tool, see Microsoft Knowledge Base Article 306460.

Logging Vulnerability - CAN-2004-0899:

A denial of service vulnerability exists that could allow an attacker to send a specially crafted DHCP message to a DHCP server. An attacker could cause the DHCP Server service to stop responding.

Mitigating Factors for Logging Vulnerability - CAN-2004-0899:

  • The DHCP Server service is not installed by default.
  • The DHCP Client service is not vulnerable to this issue.
  • DHCP Logging is not enabled by default. Only DHCP servers that have enabled DHCP Logging would be vulnerable to this issue.
  • Firewall best practices and standard default firewall configurations can help protect networks from attacks that originate outside the enterprise perimeter. Best practices recommend that systems that are connected to the Internet have a minimal number of ports exposed.

Workarounds for Logging Vulnerability - CAN-2004-0899:

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 DHCP Logging

    You can help protect against attacks that attempt to exploit this vulnerability by disabling the DHCP Logging feature. To disable this feature, perform the following steps:

    1. Start the DHCP Manager.
    2. Click the DHCP server where you want to enable logging.
    3. Click Server, and then click Properties.
    4. Click to clear the Enable DHCP Logging check box.
    5. Restart the DHCP Server service or restart the affected system.

    For more information, see Microsoft Knowledge Base Article 164524

    Impact of Workaround DHCP Logging features are disabled. It is not possible to track activity logs until this feature is enabled.

  • Block UDP port 67 and UDP port 68 at your firewall.

    These ports are used to initiate a connection with a DHCP server. Blocking these ports at the firewall will help prevent systems that are behind that firewall from being attacked by attempts to exploit this vulnerability. It is possible that other ports may be found that could be used to exploit this vulnerability. The ports that are listed are the most common attack vectors. We recommend that you block all inbound unsolicited communication from the Internet.

  • Move DHCP Services to Windows 2000 Server or a later version.

    Later versions of the DHCP Server service, such as those that are provided as part of Windows 2000 Server or Windows Server 2003 are not vulnerable to this issue. Note Windows NT 4.0 Server is nearing the end of its support life cycle on December 30, 2004. For more information about the Windows Product Lifecycle, visit the following Microsoft Support Lifecycle Web site.

FAQ for Logging Vulnerability - CAN-2004-0899:

What is the scope of the vulnerability?
Under the most likely attack scenario this is a denial of service vulnerability. An attacker who successfully exploited this vulnerability could cause the DHCP Server service to fail. Restarting the DHCP Server service will allow the service to function correctly. However, the DHCP Server service could remain vulnerable to another denial of service attack.

What causes the vulnerability?
An unchecked buffer in the method that DHCP uses to validate a value from specially crafted network packets.

What is DHCP?
Dynamic Host Configuration Protocol (DHCP) is an IP standard that is designed to reduce the complexity of administering address configurations. DHCP does this by using a server computer to centrally manage IP addresses and other related configuration details used on your network. Windows NT 4.0 Server provides the DHCP Server service, which enables the server computer to perform as a DHCP Server and to provide configuration settings to DHCP-enabled client computers on your network as described in the DHCP IETF RFC 2131.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could most likely cause DHCP to stop responding to all requests.

Who could exploit the vulnerability?
Any anonymous user who could deliver a specially crafted message to the affected system could attempt to exploit this vulnerability.

How could an attacker exploit the vulnerability?
An attacker could exploit this vulnerability by creating a program that could communicate with a vulnerable server through DHCP to send a specific kind of specially crafted DHCP message. Receipt of such a message could cause the vulnerable service to fail in such a way that it could cause a denial of service for that service.

What systems are primarily at risk from the vulnerability?
Only Windows NT 4.0 Server systems that have been configured as DHCP servers with DHCP logging enabled are vulnerable.

Could the vulnerability be exploited over the Internet?
Yes. An attacker could attempt to exploit this vulnerability over the Internet. Firewall best practices and standard default firewall configurations can help protect against attacks that originate from the Internet. Microsoft has provided information about how you can help protect your PC. IT Professionals can visit the Security Center Web site.

What does the update do?
The update removes the vulnerability by modifying the way that the DHCP Server service 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.

DHCP Request Vulnerability - CAN-2004-0900:

A remote code execution vulnerability exists that could allow an attacker to send a specially crafted DHCP message to a DHCP server. However, attempts to exploit this vulnerability would most likely result in a denial of service of the DHCP Server service.

Mitigating Factors for DHCP Request Vulnerability - CAN-2004-0900:

  • The DHCP Server service is not installed by default.
  • The DHCP Client service is not vulnerable to this issue.
  • Firewall best practices and standard default firewall configurations can help protect networks from attacks that originate outside the enterprise perimeter. Best practices recommend that systems that are connected to the Internet have a minimal number of ports exposed.

Workarounds for DHCP Request Vulnerability - CAN-2004-0900:

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.

  1. Block UDP port 67 and UDP port 68 at your firewall.

    These ports are used to initiate a connection with a DHCP server. Blocking these ports at the firewall will help prevent systems that are behind that firewall from being attacked by attempts to exploit this vulnerability. It is possible that other ports may be found that could be used to exploit this vulnerability. The ports that are listed are the most common attack vectors. We recommend that you block all inbound unsolicited communication from the Internet.

  2. Move DHCP Services to Windows 2000 Server or a later version.

    Later versions of the DHCP Server service, such as those that are provided as part of Windows 2000 Server or Windows Server 2003 are not vulnerable to this issue. Note Windows NT 4.0 Server is nearing the end of its support life cycle on December 30, 2004. For more information about the Windows Product Lifecycle, visit the following Microsoft Support Lifecycle Web site.

FAQ for DHCP Request Vulnerability - CAN-2004-0900:

What is the scope of the vulnerability?
This is a remote code execution vulnerability. An attacker who successfully exploited this vulnerability could remotely take complete control of an affected system, including installing programs; viewing, changing, or deleting data; or creating new accounts that have full privileges. However, under the most likely attack scenario this is a denial of service vulnerability. An attacker who successfully exploited this vulnerability could cause the DHCP Server service to fail. Restarting the DHCP Server service will allow the service to function correctly. However, the DHCP Server service could remain vulnerable to another denial of service attack.

What causes the vulnerability?
An unchecked buffer in the method that DHCP users validate a value from specially crafted network packets.

What is DHCP?
Dynamic Host Configuration Protocol (DHCP) is an IP standard that is designed to reduce the complexity of administering address configurations. DHCP does this by using a server computer to centrally manage IP addresses and other related configuration details used on your network. Windows NT 4.0 Server provides the DHCP Server service, which enables the server computer to perform as a DHCP Server and to provide configuration settings to DHCP-enabled client computers on your network as described in the DHCP IETF RFC 2131.

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?
Any anonymous user who could deliver a specially crafted message to the affected system could attempt to exploit this vulnerability.

How could an attacker exploit the vulnerability?
An attacker could exploit this vulnerability by creating a program that could communicate with a vulnerable server through DHCP to send a specific kind of specially crafted DHCP message. Receipt of such a message could cause the vulnerable service to fail in such a way that it could allow code execution or cause a denial of service for that service.

What systems are primarily at risk from the vulnerability?
Only Windows NT 4.0 Server systems that have been configured as DHCP servers are vulnerable.

Could the vulnerability be exploited over the Internet?
Yes. An attacker could attempt to exploit this vulnerability over the Internet. Firewall best practices and standard default firewall configurations can help protect against attacks that originate from the Internet. Microsoft has provided information about how you can help protect your PC. IT Professionals can visit the Security Center Web site.

What does the update do?
The update removes the vulnerability by modifying the way that the DHCP Server service 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.

Installation Platforms and Prerequisites:

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

Windows NT 4.0 (all versions)

Prerequisites
This security update requires 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. For more information about the terminology that appears in this bulletin, such as hotfix, see Microsoft Knowledge Base Article 824684.

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-kb885249-x86-enu /q

For Windows NT Server 4.0 Terminal Server Edition:

Windowsnt4terminalserver-kb885249-x86-enu /q

To install the security update without forcing the system to restart, manually stop the DHCP Server Service and then use the following command at a command prompt for Windows NT Server 4.0:

Windowsnt4server-kb885249-x86-enu /z /q

For Windows NT Server 4.0 Terminal Server Edition:

Windowsnt4terminalserver-kb885249-x86-enu /z /q

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

Restart Requirement

You must restart your system after you apply this security update. However, this restart may be avoided if you follow the instructions provided in the Deployment Information section of this security update.

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%\$NTUninstallKB885249$ 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, file name, or size information could change during installation. See the Verifying Update Installation section for details about how to verify an installation.

Windows NT Server 4.0:

File NameVersionDateTimeSize
Dhcpssvc.dll4.0.1381.730415-Oct-200414:49128,272

Windows NT Server 4.0 Terminal Server Edition:

File NameVersionDateTimeSize
Dhcpssvc.dll4.0.1381.3358729-Sep-200419:39128,272

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.

    1. Click Start, and then click Search.
    2. In the Search Results pane, click All files and folders under Search Companion.
    3. In the All or part of the file name box, type a file name from the appropriate file information table, and then click Search.
    4. 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.

    5. 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\KB885249\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 885249 security update into the Windows installation source files.

Other Information

Acknowledgments

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

  • Kostya Kortchinsky from CERT RENATER for reporting the Logging Vulnerability (CAN-2004-0899) and the DHCP Request Vulnerability (CAN-2004-0900).

Obtaining Other Security Updates:

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

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:

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, visit 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, visit 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 (December 14, 2004): Bulletin published

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