Microsoft Security Bulletin MS05-028

A remote code execution vulnerability exists in the way that Windows processes Web Client requests that could allow an attacker who successfully exploited this vulnerable to take complete control of the affected system.

•	An attacker must have valid logon credentials to exploit this vulnerability. The vulnerability could not be exploited by anonymous users.
•	By default, the Web Client service is disabled in Windows Server 2003. An administrator would have to manually enable this service for the system to become vulnerable to this issue.
•	Attempts to exploit this vulnerability would most likely result in a denial of service condition.
•	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. By default, the Internet Connection Firewall that is provided as part of Windows XP Service Pack 1 and Windows Server 2003 blocks the affected ports from responding to network-based attempts to exploit this vulnerability.
•	The Windows Server 2003 Security Guide recommends disabling the Web Client service. Environments that comply with these guidelines would be at a reduced risk from this vulnerability.

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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 in the following section.

•	Disable the Web Client service Disabling the Web Client service will help protect the affected system from attempts to exploit this vulnerability. To disable the Web Client service, follow these steps: 1. Click Start, and then click Control Panel. Alternatively, point to Settings, and then click Control Panel. 2. Double-click Administrative Tools. 3. Double-click Services. 4. Double-click WebClient. 5. In the Startup type list, click Disabled. 6. Click Stop, and then click OK. You can also stop and disable the Web Client service by using the following command at the command prompt: sc stop WebClient & sc config WebClient start= disabled Impact of Workaround: If the Web Client service is disabled, Web Distributed Authoring and Versioning (WebDAV) requests are not transmitted. If the Web Client service is disabled, any services that explicitly depend on the Web Client service will not start, and an error message will be logged in the System log. Windows Server 2003 users will not be able to use the "Open as Web Folder" functionality.
•	Use the Group Policy settings to disable the WebClient service on all affected systems that do not require this feature. Because the Web Client service is a possible attack vector, disable the service by using the Group Policy settings. You can disable the startup of this service at either the local, site, domain, or organizational-unit level by using Group Policy object functionality in Windows 2000 domain environments or in Windows Server 2003 domain environments. Note You may also review the Windows Server 2003 Security Guide. This guide includes information about how to disable services. For more information about Group Policy, visit the following Microsoft Web sites: • Step-by-Step Guide to Understanding the Group Policy Feature Set • Windows 2000 Group Policy • Group Policy in Windows Server 2003 Impact of Workaround: If the Web Client service is disabled, Web Distributed Authoring and Versioning (WebDAV) requests are not transmitted. If the Web Client service is disabled, any services that explicitly depend on the Web Client service will not start, and an error message will be logged in the System log. Windows Server 2003 users will not be able to use the "Open as Web Folder" functionality.
•	Block TCP ports 139 and 445 at the firewall: Although WebDAV uses TCP port 80 for outbound communication, TCP ports 139 and 445 can be used inbound to attempt to connect to this service and try to exploit this vulnerability. Blocking them at the firewall can help prevent systems that are behind that firewall from attempts to exploit this vulnerability. We recommend that you block all unsolicited inbound communication from the Internet to help prevent attacks that may use other ports. For more information about ports, visit the following Web site.
•	To help protect from network-based attempts to exploit this vulnerability, use a personal firewall, such as the Internet Connection Firewall, which is included with Windows XP and with Windows Server 2003. By default, the Internet Connection Firewall feature in Windows XP and in Windows Server 2003 helps protect your Internet connection by blocking unsolicited incoming traffic. We recommend that you block all unsolicited incoming communication from the Internet. To enable the Internet Connection Firewall feature by using the Network Setup Wizard, follow these steps: 1. Click Start, and then click Control Panel. 2. In the default Category View, click Network and Internet Connections, and then click Setup or change your home or small office network. The Internet Connection Firewall feature is enabled when you select a configuration in the Network Setup Wizard that indicates that your system is connected directly to the Internet. To configure Internet Connection Firewall manually for a connection, follow these steps: 1. Click Start, and then click Control Panel. 2. In the default Category View, click Networking and Internet Connections, and then click Network Connections. 3. Right-click the connection on which you want to enable Internet Connection Firewall, and then click Properties. 4. Click the Advanced tab. 5. Click to select the Protect my computer or network by limiting or preventing access to this computer from the Internet check box, and then click OK. Note If you want to enable certain programs and services to communicate through the firewall, click Settings on the Advanced tab, and then select the programs, the protocols, and the services that are required.
•	To help protect from network-based attempts to exploit this vulnerability, enable advanced TCP/IP filtering on systems that support this feature. You can enable advanced TCP/IP filtering to block all unsolicited inbound traffic. For more information about how to configure TCP/IP filtering, see Microsoft Knowledge Base Article 309798.
•	To help protect from network-based attempts to exploit this vulnerability, block the affected ports by using IPSec on the affected systems. Use Internet Protocol security (IPSec) to help protect network communications. Detailed information about IPSec and about how to apply filters is available in Microsoft Knowledge Base Article 313190 and Microsoft Knowledge Base Article 813878.

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What is the scope of the vulnerability?
This is a remote code execution vulnerability. An attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. To attempt to exploit the vulnerability, an attacker must have valid logon credentials. The vulnerability could not be exploited by anonymous users. An attacker could also use this vulnerability to perform a local elevation of privilege attack.

What causes the vulnerability?
An unchecked buffer in the WebClient service.

What is the Web Client service?
The Web Client service allows applications to access documents on the Internet. Web Client extends the networking capability of Windows by allowing standard Win32 applications to create, read, and write files on Internet file servers by using the WebDAV protocol. The WebDAV protocol is a file-access protocol that is described in XML and that travels over the Hypertext Transfer Protocol (HTTP). By using standard HTTP, WebDAV runs over the existing Internet infrastructure. For example, WebDAV runs over firewalls and routers.

If the Web Client service is stopped, you will be prevented from using the Web Publishing Wizard to publish data to the Internet for locations that use the WebDAV protocol. If this service is disabled, any services that explicitly depend on this service will not start. For more information on WebDAV, see the following product documentation.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

Who could exploit the vulnerability?
To attempt to exploit the vulnerability, an attacker must have valid logon credentials. The vulnerability could not be exploited by anonymous users. Even though the Web Client service is used to support the WebDAV protocol over the Internet, an authenticated attacker must perform the steps that are required to attempt to exploit this issue. If the Guest account has been enabled on an affected system, then this attack could be performed by any user.

How could an attacker exploit the vulnerability?
There are several different ways that an attacker could try to exploit this vulnerability. An attacker would first have to authenticate to the system. An attacker could then try to exploit the vulnerability directly over a network by creating a series of specially crafted messages and sending them to an affected system. The messages could then cause the affected system to execute code.

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 either locally or remotely.

What systems are primarily at risk from the vulnerability?
All affected operating systems are at risk from this vulnerability. By default, the Internet Connection Firewall that is provided as part of Windows XP Service Pack 1 and Windows Server 20003 blocks the affected ports from responding to network-based attempts to exploit this vulnerability. By default, the Web Client service is disabled in Windows Server 2003.

Could the vulnerability be exploited over the Internet?
Yes. An attacker could try 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. End users can visit the Protect Your PC Web site. 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 Web Client 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 to indicate 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 to indicate 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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Prerequisites
This security update requires the originally released version of Windows Server 2003.

Inclusion in Future Service Packs:
The update for this issue is included in Service Pack 1.

Installation Information

This security update supports the following setup switches.

Note You can combine these switches into one command. For backward compatibility, the security update also supports many of the setup switches that the earlier version of the Setup program uses. For more information about the supported installation switches, see Microsoft Knowledge Base Article 262841. For more information about the Update.exe installer, visit the Microsoft TechNet Web site.

Deployment Information

To install the security update without any user intervention, use the following command at a command prompt for Windows Server 2003:

Windowsserver2003-kb896426-x86-enu /quiet

Note Use of the /quiet switch will suppress all messages. This includes suppressing failure messages. Administrators should use one of the supported methods to verify the installation was successful when they use the /quiet switch. Administrators should also review the KB896426.log file for any failure messages when they use this switch.

To install the security update without forcing the system to restart, use the following command at a command prompt for Windows Server 2003:

Windowsserver2003-kb896426-x86-enu /norestart

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

Restart Requirement

You must restart your system after you apply this security update. For more information about the reasons why you may be prompted to restart your computer, see Microsoft Knowledge Base Article 887012.

Removal Information

To remove this 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%\$NTUninstallKB896426$\Spuninst folder.

File Information

The English version of this security update has the file attributes 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.

Windows Server 2003, Web Edition; Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; Windows Server 2003, Datacenter Edition; and Windows Small Business Server 2003:

Windows Server 2003, Enterprise Edition for Itanium-based Systems and Windows Server 2003, Datacenter Edition for Itanium-based Systems:

Note When you install this security update on Windows Server 2003, the installer checks to see if one or more of the files that are being updated on your system have previously been updated by a Microsoft hotfix. If you have previously installed a hotfix to update an affected file, the installer copies the RTMQFE files to your system. Otherwise, the installer copies the RTMGDR files to your system.

For more information about this behavior, see Microsoft Knowledge Base Article 824994.

For more information about the Update.exe installer, visit the Microsoft TechNet Web site.

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

Verifying that the Update Has Been Applied

Prerequisites
This security update requires Microsoft Windows XP Service Pack 1. For more information, see Microsoft Knowledge Base Article 322389.

Inclusion in Future Service Packs:
The update for this issue is included in Windows XP Service Pack 2.

Installation Information

This security update supports the following setup switches.

Note You can combine these switches into one command. For backward compatibility, the security update also supports the setup switches that the earlier version of the Setup program uses. For more information about the supported installation switches, see Microsoft Knowledge Base Article 262841. For more information about the Update.exe installer, visit the Microsoft TechNet Web site.

Deployment Information

To install the security update without any user intervention, use the following command at a command prompt for Microsoft Windows XP:

Windowsxp-kb896426-x86-enu /quiet

Note Use of the /quiet switch will suppress all messages. This includes suppressing failure messages. Administrators should use one of the supported methods to verify the installation was successful when they use the /quiet switch. Administrators should also review the KB896426.log file for any failure messages when they use this switch.

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

Windowsxp-kb896426-x86-enu /norestart

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

Restart Requirement

You must restart your system after you apply this security update. For more information about the reasons why you may be prompted to restart your computer, see Microsoft Knowledge Base Article 887012.

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%\$NTUninstallKB896426$\Spuninst folder.

File Information

The English version of this security update has the file attributes 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.

Windows XP Home Edition Service Pack 1, Windows XP Professional Service Pack 1, Windows XP Tablet PC Edition, and Windows XP Media Center Edition:

Windows XP 64-Bit Edition Service Pack 1 (Itanium):

Windows XP 64-Bit Edition Version 2003 (Itanium):

Notes When you install these security updates, the installer checks to see if one or more of the files that are being updated on your system have previously been updated by a Microsoft hotfix.

If you have previously installed a hotfix to update one of these files, the installer copies the RTMQFE files to your system. Otherwise, the installer copies the RTMGDR files to your system.

For more information about this behavior, see Microsoft Knowledge Base Article 824994.

For more information about the Update.exe installer, visit the Microsoft TechNet Web site.

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

Note For Windows XP 64-Bit Edition Version 2003 (Itanium), this security update is the same as the Windows Server 2003 for Itanium-based Systems security update.

Verifying that the Update Has Been Applied