Microsoft Security Bulletin MS03-049

Technical description:

A security vulnerability exists in the Workstation service that could allow remote code execution on an affected system. This vulnerability results because of an unchecked buffer in the Workstation service.

If exploited, an attacker could gain System privileges on an affected system, or could cause the Workstation service to fail. An attacker could take any action on the system, including installing programs, viewing data, changing data, or deleting data, or creating new accounts with full privileges.

Mitigating factors:

• If users have blocked inbound UDP ports 138, 139, 445 and TCP ports 138, 139, 445 by using a firewall an attacker would be prevented from sending messages to the Workstation service. Most firewalls, including Internet Connection Firewall in Windows XP, block these ports by default.
• Disabling the Workstation service will prevent the possibility of attack. However there are a number of impacts when performing this workaround. Please see the Workaround section for more details.
• Only Windows 2000 and Window XP are affected. Other operating systems are not vulnerable to this attack.

Severity Rating:

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

Vulnerability identifier: CAN-2003-0812

Microsoft has tested the following workarounds that apply to this vulnerability. These workarounds help block known attack vectors, however they will not correct the underlying vulnerability. Workarounds may reduce functionality in some cases; in such cases, the reduction in functionality is identified below.

• Block UDP ports 138, 139, 445 and TCP ports 138, 139, 445 at your firewall.

  These ports are used to accept a Remote Procedure Call (RPC) connection at a remote computer. Blocking them at the firewall will help prevent systems behind that firewall from being attacked by attempts to exploit this vulnerability.

• Use a personal firewall such as Internet Connection Firewall, which is included with Windows XP.

  If you use the Internet Connection Firewall feature in Windows XP to help protect your Internet connection, Internet Connection Firewall blocks inbound traffic from the Internet or from the intranet by default.

  To enable the Internet Connection Firewall feature by using the Network Setup Wizard:

  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 computer is connected directly to the Internet.

  To configure Internet Connection Firewall manually for a connection:

  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. 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 the use of some applications and services through the firewall, click Settings on the Advanced tab, and then select the programs, the protocols, and the services.

• Enable advanced TCP/IP filtering on Windows 2000-based systems and on Windows XP-based systems.

  You can enable advanced TCP/IP filtering to block all unsolicited, inbound traffic. For additional information about how to configure TCP/IP filtering, click the following article number to view the article in the Microsoft Knowledge Base:

  309798 HOW TO: Configure TCP/IP Filtering in Windows 2000

• Disable the Workstation service.

  You can disable the Workstation service to help prevent the possibility of an attack.

  To disable the Workstation service on Windows XP:

  • Click Start, and then click Control Panel.
  • In the default Category View, click Performance and Maintenance.
  • Click Administrative Tools.
  • Double-click Services.
  • Double-click Workstation.
  • On the General tab, click Disabled in the Startup type list.
  • Click Stop under Service status, and then click OK.

  To disable the Workstation service on Windows 2000:

  • Click Start, point to Settings, and then click Control Panel.
  • Double-click Administrative Tools.
  • Double-click Services.
  • Double-click Workstation.
  • On the General tab, click Disabled in the Startup type list.
  • Click Stop under Service status, and then click OK.

  Impact of Workaround: If the Workstation service is disabled, the system cannot connect to any shared file resources or shared print resources on a network. Only use this workaround on stand-alone systems (such as many home systems) that do not connect to a network. If the Workstation service is disabled, any services that explicitly depend on the Workstation service do not start, and an error message is logged in the system event log. The following services depend on the Workstation service:

  • Alerter
  • Browser
  • Messenger
  • Net Logon
  • RPC Locator

  These services are required to access resources on a network and to perform domain authentication. Internet connectivity and browsing for stand-alone systems, such as users on dial-up connections, on DSL connections, or on cable modem connections, should not be affected if these services are disabled.

  Note: The Microsoft Baseline Security Analyzer will not function if the Workstation service is disabled. It is possible that other applications may also require the Workstation service. If an application requires the Workstation service, simply re-enable the service. This can be performed by changing the Startup Type for the Workstation service back to Automatic and restarting the system.

What is the scope of the vulnerability?
This is a buffer overrun vulnerability. An attacker who successfully exploited this vulnerability could allow remote code execution with System privileges on an affected system, or an attacker could cause the Workstation service to fail. The attacker could then take any action on the system, including installing programs, viewing data, changing data, or deleting data, or creating new accounts with full privileges. 

What causes the vulnerability?
The vulnerability results because of an unchecked buffer within the Workstation service.

What is the Workstation Service?
Both local file system requests and remote file or print network requests are routed through the Workstation service. This service determines where the resource is located and then routes the request to the local file system or to the networking components. When the Workstation service is stopped, all requests are assumed to be local requests. For a detailed understanding of the Windows networking architecture, visit the following Microsoft Web site: http://www.microsoft.com /technet/prodtechnol/windows2000serv/reskit/cnet/cnad_arc_tfgi.mspx

What could this vulnerability enable an attacker to do?
An attacker who successfully exploited this vulnerability could cause code to execute with System privileges on an affected system or could cause the Workstation service to fail. An attacker could then take any action on the system, including installing programs, viewing data, changing data, or deleting data, or creating new accounts with full privileges.

Who could exploit the vulnerability?
Any anonymous user who could deliver a malformed message to the Workstation service on an affected system could attempt to exploit this vulnerability. Because the Workstation service is enabled by default in all versions of Windows, this means that any user who could establish a connection with an affected system could attempt to exploit this vulnerability.

How could an attacker exploit this vulnerability?
An attacker could seek to exploit this vulnerability by creating a specially-crafted network message and by sending the message to the Workstation service on an affected system. Receipt of such a message could cause the Workstation service on the vulnerable system to fail in such a way that could allow the Workstation service to execute code.
An attacker could also access the affected component through another vector, such as one that would involve logging onto the system interactively or by using another application that passed parameters to the vulnerable component (locally or remotely).

What does the update do?
The update eliminates the vulnerability by ensuring that the Workstation service properly validates the length of a message before passing the message to the allocated buffer.

Why does the Windows XP update reference the MS03-043 Security Bulletin?
There is no Windows XP version of the MS03-049 security update. The Windows XP security updates that released as part of Security Bulletin MS03-043 (828035) include the updated file that helps protect from this vulnerability. The Windows XP download links in this bulletin link to the Windows XP MS03-043 security update. If you have applied the Windows XP security updates for MS03-043 (828035) you do not have to reapply the update to be protected against the vulnerability described in this bulletin. However, the Windows 2000 security update that is released as part of this security bulletin contains updated files that were not part of the MS03-043 (828035) security bulletin. Customers have to apply this Windows 2000 security update even if they applied the Windows 2000 security updates for MS03-043 (828035). Future updates to the MS03-043 Windows XP security update may be released, they will also contain the necessary files to be protected against this vulnerability.

Installation platforms and Prerequisites:

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