Microsoft Security Bulletin MS09-044 - Critical

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability. The following mitigating factors may be helpful in your situation:

• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• In a Web-based attack scenario, an attacker could host a Web site that contains a Web page that is used to exploit this vulnerability. In addition, compromised Web sites and Web sites that accept or host user-provided content or advertisements could contain specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to visit these Web sites. Instead, an attacker would have to persuade users to visit the Web site, typically by getting them to click a link in an e-mail message or Instant Messenger message that takes users to the attacker's Web site.
• RDP servers are not affected by this vulnerability. Only RDP clients are affected.
• In RDP Version 6.0 and RDP Version 6.1, the vulnerability cannot be exploited by a malicious Web site that invokes the RDP ActiveX control, which connects the session to a malicious RDP server.

Workaround refers to a setting or configuration change that does not correct the underlying vulnerability but would help block known attack vectors before you apply the update. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:

• Restrict access to mstscax.dll

  Note You must be an administrator to use these commands.

  Windows XP and Windows Server 2003

  Run the following commands from a command prompt:

  cacls %windir%\system32\mstscax.dll /E /P everyone:N
cacls %windir%\sysWOW64\mstscax.dll /E /P everyone:N

  Windows Vista and Windows Server 2008

  Run the following commands from an elevated command prompt:

  takeown /F %windir%\system32\mstscax.dll
cacls %windir%\system32\mstscax.dll /E /P everyone:N
takeown /F %windir%\SysWOW64\mstscax.dll
cacls %windir%\SysWOW64\mstscax.dll /E /P everyone:N
for /F "tokens=*" %G IN ('dir /b /s %windir%\winsxs\mstscax.dll') DO takeown /F %G && cacls %G /E /P everyone:N

  Impact of workaround.After performing these steps, you will not be able to make outbound Remote Desktop connections.

  How to undo the workaround

  Note You must be an administrator to use these commands.

  Windows XP and Windows Server 2003

  Run the following commands from a command prompt:

  cacls %windir%\system32\mstscax.dll /E /R everyone
cacls %windir%\SysWOW64\mstscax.dll /E /R everyone

  Windows Vista and Windows Server 2008

  Run the following commands from an elevated command prompt:

  cacls %windir%\system32\mstscax.dll /E /R everyone
cacls %windir%\SysWOW64\mstscax.dll /E /R everyone
for /F "tokens=*" %G IN ('dir /b /s %windir%\winsxs\mstscax.dll') DO cacls %G /E /R everyone

• Prevent the Remote Desktop Connection ActiveX control from running in Internet Explorer

  You can disable attempts to instantiate a COM object in Internet Explorer by setting the kill bit for the control in the registry.

  Note This workaround prevents exploitation through the Web-based attack vector, but it does not protect the user against manually initiating a connection to a malicious RDP server.

  Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

  For detailed steps that you can use to prevent an ActiveX control from running in Internet Explorer, see Microsoft Knowledge Base Article 240797.

  Follow these steps to create a Compatibility Flags value in the registry to prevent a COM object from being instantiated in Internet Explorer.

  1. Create a text file named Disable_Remote_Desktop_ActiveX.reg with the following contents:
     
     Windows Registry Editor Version 5.00
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{7390f3d8-0439-4c05-91e3-cf5cb290c3d0}]
     "Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\ActiveX Compatibility\{7390f3d8-0439-4c05-91e3-cf5cb290c3d0}]"Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{4EB89FF4-7F78-4A0F-8B8D-2BF02E94E4B2}] "Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\ActiveX Compatibility\{4EB89FF4-7F78-4A0F-8B8D-2BF02E94E4B2}]"Compatibility Flags"=dword:00000400
  2. Apply this .reg file to individual systems by double-clicking it. You can also apply it across domains by using Group Policy.

  For more information about Group Policy, visit the following Microsoft Web sites:

  Group Policy collection

  What is Group Policy Object Editor?

  Core Group Policy Tools and Settings

  Note You must restart Internet Explorer for your changes to take effect.

  Impact of workaround. Users will not be able to start remote desktop connections from within Web pages.

  How to undo the workaround. Delete the registry keys previously added in implementing this workaround.

What is the scope of the vulnerability? 
This vulnerability could allow remote code execution. If a user is logged on with administrative privileges, 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 privileges. Users whose accounts are configured to have fewer privileges on the system could be less impacted than users who operate with administrative privileges.

What causes the vulnerability? 
A remote code execution vulnerability exists in the way that the Microsoft Remote Desktop Connection processes specific parameters returned by the RDP server. This issue results in a heap overflow on the client that can be leveraged by the attacker to execute arbitrary code with the privileges of the logged-on user.

What is the Microsoft Remote Desktop Protocol? 
The Microsoft Remote Desktop Protocol (RDP) provides remote display and input capabilities over network connections for Microsoft Windows-based applications running on a server. RDP is based on, and is an extension of, the ITU T.120 family of protocols. RDP is a multiple-channel capable protocol that allows for separate virtual channels for carrying device communication and presentation data from the server, as well as encryption of the session. RDP provides an extensible base and supports up to 64,000 separate channels for data transmission and provisions for multipoint transmission. On some platforms, Microsoft Remote Desktop Connection is called Terminal Services Client.

Microsoft Windows implements RDP as follows:

• Windows 2000: Terminal Services includes enhanced RDP Version 5.0.
• Windows XP Service Pack 2: Uses RDP Version 5.1 for Remote Desktop Connection and for Remote Assistant. Windows XP also includes Remote Desktop Web Connection, which is an updated version of the Terminal Services Advanced Client (TSAC), an RDP client based on a Microsoft ActiveX control. Remote Desktop Web Connection supports RDP Version 5.1 and is backward compatible with RDP Version 5.0. In addition, two out-of-box updates are available which implement RDP Version 6.0 and RDP Version 6.1 on Windows XP Service Pack 2 respectively.
• Windows XP Service Pack 3: Uses RDP Version 5.2 for Remote Desktop Connection and for Remote Assistance.
• Windows Server 2003 Service Pack 2: Uses RDP Version 5.2 for Remote Desktop Connection and for Remote Assistant. Remote Desktop Web Connection supports RDP Version 5.2 and is backward compatible with RDP Versions 5.1 and 5.0. In addition, an out-of-box update is available which implements RDP Version 6.0 on these platforms.
• Windows Vista: Uses RDP Version 6.0 for Remote Desktop Connection.
• Windows Server 2008, Windows Vista Service Pack 1, and Windows Vista Service Pack 2: Uses RDP Version 6.1 for Remote Desktop Connection.

Why is this vulnerability rated Critical for all versions of RDP with the exception of RDP Version 6.0 and RDP Version 6.1? 
This vulnerability is rated Critical for RDP Version 5.0, RDP Version 5.1, and RDP Version 5.2 because of a specific attack scenario applicable to these RDP versions. In this attack scenario, an attacker could host a specially crafted Web site that is designed to exploit this vulnerability. Such a Web site could invoke the RDP ActiveX control without further user interaction after visiting the Web site and cause the control to connect the session to a malicious RDP server. On RDP Version 6.0 and RDP Version 6.1, exploitation requires the user to accept the connection by interacting with the user interface.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability could run arbitrary code as the logged-on user. If a user is logged on with administrative user rights, an attacker could take complete control of the affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

How could an attacker exploit the vulnerability? 
An attacker could host a specially crafted Web site and then convince a user to view the Web site. The Web site would then invoke the RDP Client ActiveX control to connect the session to a malicious RDP server. An attacker could also convince a user to manually connect to a remote RDP server in order to exploit the vulnerability.

What systems are primarily at risk from the vulnerability? 
This vulnerability requires that a user be logged on and visiting a Web site or engaged in a remote connection with another system for any malicious action to occur. Therefore, workstations or clients intended to connect to terminal servers are particularly affected by this vulnerability.

What does the update do? 
This security update addresses the vulnerability by changing the way that the Remote Desktop Connection deals with unexpected parameters sent by the RDP server.

When this security bulletin was issued, had this vulnerability been publicly disclosed? 
No, 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.

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability. The following mitigating factors may be helpful in your situation:

• By default, all supported versions of Microsoft Outlook and Microsoft Outlook Express open HTML e-mail messages in the Restricted sites zone. The Restricted sites zone helps mitigate attacks that could try to exploit this vulnerability by preventing Active Scripting and ActiveX controls from being used when reading HTML e-mail messages. However, if a user clicks a link in an e-mail message, the user could still be vulnerable to exploitation of this vulnerability through the Web-based attack scenario.
• By default, Internet Explorer on Windows Server 2003 and Windows Server 2008 runs in a restricted mode that is known as Enhanced Security Configuration. This mode sets the security level for the Internet zone to High. This is a mitigating factor for Web sites that you have not added to the Internet Explorer Trusted sites zone. See the FAQ subsection of this vulnerability section for more information about Internet Explorer Enhanced Security Configuration.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• In a Web-based attack scenario, an attacker could host a Web site that contains a Web page that is used to exploit this vulnerability. In addition, compromised Web sites and Web sites that accept or host user-provided content or advertisements could contain specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to visit these Web sites. Instead, an attacker would have to persuade users to visit the Web site, typically by getting them to click a link in an e-mail message or Instant Messenger message that takes users to the attacker's Web site.
• RDP Version 5.0, RDP Version 5.1, RDP Version 5.2, RDP Version 6.0, and Remote Desktop Connection Client for Mac 2.0 are not affected by this vulnerability.
• RDP servers are not affected by this vulnerability. Only RDP clients are affected.

Workaround refers to a setting or configuration change that does not correct the underlying vulnerability but would help block known attack vectors before you apply the update. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:

• Prevent the Remote Desktop Connection ActiveX control from running in Internet Explorer

  You can disable attempts to instantiate a COM object in Internet Explorer by setting the kill bit for the control in the registry.

  Note This workaround prevents exploitation through the Web-based attack vector, but it does not protect the user against manually initiating a connection to a malicious RDP server.

  Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

  For detailed steps that you can use to prevent an ActiveX control from running in Internet Explorer, see Microsoft Knowledge Base Article 240797.

  Follow these steps to create a Compatibility Flags value in the registry to prevent a COM object from being instantiated in Internet Explorer.

  1. Create a text file named Disable_Remote_Desktop_ActiveX.reg with the following contents:
     
     Windows Registry Editor Version 5.00
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{7390f3d8-0439-4c05-91e3-cf5cb290c3d0}]
     "Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\ActiveX Compatibility\{7390f3d8-0439-4c05-91e3-cf5cb290c3d0}]"Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{4EB89FF4-7F78-4A0F-8B8D-2BF02E94E4B2}] "Compatibility Flags"=dword:00000400
     
     [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\ActiveX Compatibility\{4EB89FF4-7F78-4A0F-8B8D-2BF02E94E4B2}]"Compatibility Flags"=dword:00000400
  2. Apply this .reg file to individual systems by double-clicking it. You can also apply it across domains by using Group Policy.

  For more information about Group Policy, visit the following Microsoft Web sites:

  Group Policy collection

  What is Group Policy Object Editor?

  Core Group Policy Tools and Settings

  Note You must restart Internet Explorer for your changes to take effect.

  Impact of workaround. Users will not be able to start remote desktop connections from within Web pages.

  How to undo the workaround. Delete the registry keys previously added in implementing this workaround.

• Set Internet and Local intranet security zone settings to "High" to prompt before running ActiveX Controls and Active Scripting in these zones

  You can help protect against exploitation of this vulnerability by changing your settings for the Internet security zone to prompt before running ActiveX controls and Active Scripting. You can do this by setting your browser security to High.

  To raise the browsing security level in Internet Explorer, follow these steps:

  1. On the Internet Explorer Tools menu, click Internet Options.
  2. In the Internet Options dialog box, click the Security tab, and then click the Internet icon.
  3. Under Security level for this zone, move the slider to High. This sets the security level for all Web sites you visit to High.

  Note If no slider is visible, click Default Level, and then move the slider to High.

  Note Setting the level to High may cause some Web sites to work incorrectly. If you have difficulty using a Web site after you change this setting, and you are sure the site is safe to use, you can add that site to your list of trusted sites. This will allow the site to work correctly even with the security setting set to High.

  Impact of workaround. There are side effects to prompting before running ActiveX Controls and Active Scripting. Many Web sites that are on the Internet or on an intranet use ActiveX or Active Scripting to provide additional functionality. For example, an online e-commerce site or banking site may use ActiveX Controls to provide menus, ordering forms, or even account statements. Prompting before running ActiveX Controls or Active Scripting is a global setting that affects all Internet and intranet sites. You will be prompted frequently when you enable this workaround. For each prompt, if you feel you trust the site that you are visiting, click Yes to run ActiveX Controls or Active Scripting. If you do not want to be prompted for all these sites, use the steps outlined in "Add sites that you trust to the Internet Explorer Trusted sites zone".

  Add sites that you trust to the Internet Explorer Trusted sites zone

  After you set Internet Explorer to require a prompt before it runs ActiveX controls and Active Scripting in the Internet zone and in the Local intranet zone, you can add sites that you trust to the Internet Explorer Trusted sites zone. This will allow you to continue to use trusted Web sites exactly as you do today, while helping to protect you from this attack on untrusted sites. We recommend that you add only sites that you trust to the Trusted sites zone.

  To do this, follow these steps:

  1. In Internet Explorer, click Tools, click Internet Options, and then click the Security tab.
  2. In the Select a Web content zone to specify its current security settings box, click Trusted Sites, and then click Sites.
  3. If you want to add sites that do not require an encrypted channel, click to clear the Require server verification (https:) for all sites in this zone check box.
  4. In the Add this Web site to the zone box, type the URL of a site that you trust, and then click Add.
  5. Repeat these steps for each site that you want to add to the zone.
  6. Click OK two times to accept the changes and return to Internet Explorer.

  Note Add any sites that you trust not to take malicious action on your system. Two in particular that you may want to add are *.windowsupdate.microsoft.com and *.update.microsoft.com. These are the sites that will host the update, and it requires an ActiveX Control to install the update.

What is the scope of the vulnerability? 
This vulnerability could allow remote code execution. If a user is logged on with administrative privileges, 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 privileges. Users whose accounts are configured to have fewer privileges on the system could be less impacted than users who operate with administrative privileges.

What causes the vulnerability? 
The Remote Desktop Web Connection ActiveX control methods do not perform sufficient parameter validation.

What is the Remote Desktop Web Connection ActiveX control? 
The Remote Desktop Web Connection ActiveX control allows you to access your computer, via the Internet, from another computer using Internet Explorer.

What is the Microsoft Remote Desktop Protocol? 
The Microsoft Remote Desktop Protocol (RDP) provides remote display and input capabilities over network connections for Microsoft Windows-based applications running on a server. RDP is based on, and an extension of, the ITU T.120 family of protocols. RDP is a multiple-channel capable protocol that allows for separate virtual channels for carrying device communication and presentation data from the server, as well as encryption of the session. RDP provides an extensible base and supports up to 64,000 separate channels for data transmission and provisions for multipoint transmission. On some platforms, Microsoft Remote Desktop Connection is called Terminal Services Client.

Microsoft Windows implements RDP as follows:

• Windows 2000: Terminal Services includes enhanced RDP Version 5.0.
• Windows XP Service Pack 2: Uses RDP Version 5.1 for Remote Desktop Connection and for Remote Assistant. Windows XP also includes Remote Desktop Web Connection, which is an updated version of the Terminal Services Advanced Client (TSAC), an RDP client based on a Microsoft ActiveX control. Remote Desktop Web Connection supports RDP Version 5.1 and is backward compatible with RDP Version 5.0. In addition, two out-of-box updates are available which implement RDP Version 6.0 and RDP Version 6.1 on Windows XP Service Pack 2 respectively.
• Windows XP Service Pack 3: Uses RDP Version 5.2 for Remote Desktop Connection and for Remote Assistance.
• Windows Server 2003 Service Pack 2: Uses RDP Version 5.2 for Remote Desktop Connection and for Remote Assistant. Remote Desktop Web Connection supports RDP Version 5.2 and is backward compatible with RDP Versions 5.1 and 5.0. In addition, an out-of-box update is available which implements RDP Version 6.0 on these platforms.
• Windows Vista: Uses RDP Version 6.0 for Remote Desktop Connection.
• Windows Server 2008, Windows Vista Service Pack 1, and Windows Vista Service Pack 2: Uses RDP Version 6.1 for Remote Desktop Connection.

What is the Internet Explorer Enhanced Security Configuration?

Internet Explorer Enhanced Security Configuration is a group of preconfigured Internet Explorer settings that reduce the likelihood of a user or of an administrator downloading and running specially crafted Web content on a server. Internet Explorer Enhanced Security Configuration reduces this risk by modifying many security-related settings. This includes the settings on the Security tab and the Advanced tab in the Internet Options dialog box. Some of the important modifications include the following:

• Security level for the Internet zone is set to High. This setting disables scripts, ActiveX controls, Microsoft Java Virtual Machine (MSJVM), and file downloads.
• Automatic detection of intranet sites is disabled. This setting assigns all intranet Web sites and all Universal Naming Convention (UNC) paths that are not explicitly listed in the Local intranet zone to the Internet zone.
• Install On Demand and non-Microsoft browser extensions are disabled. This setting prevents Web pages from automatically installing components and prevents non-Microsoft extensions from running.
• Multimedia content is disabled. This setting prevents music, animations, and video clips from running.

For more information regarding Internet Explorer Enhanced Security Configuration, see the guide, Managing Internet Explorer Enhanced Security Configuration.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability could run arbitrary code on a users system. This could allow an attacker to take complete control of the affected system.

How could an attacker exploit the vulnerability? 
An attacker could host a specially crafted Web site that is designed to exploit this vulnerability through Internet Explorer and then persuade a user to view the Web site. The Web site would invoke the vulnerable ActiveX control and subsequently exploit the vulnerability.

Malicious Web sites can also include Web sites that accept user-provided content or advertisements, Web sites that host user-provided content or advertisements, and compromised Web sites. These Web sites could contain specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to visit these Web sites. Instead, an attacker would have to persuade users to visit the Web site, typically by getting them to click a link an e-mail message or in an Instant Messenger request that takes users to the attacker's Web site. It could also be possible to display specially crafted Web content by using banner advertisements or by using other methods to deliver Web content to affected systems.

What systems are primarily at risk from the vulnerability? 
This vulnerability requires that a user be logged on and visiting a Web site or engaged in a remote connection with another system for any malicious action to occur. Therefore, workstations or clients intended to connect to terminal servers are particularly affected by this vulnerability.

Clients that are being used to connect to Web-based Terminal Services (RDP) servers are particularly affected by this vulnerability, as they will have the control installed and operational. Other clients may be affected if they have the control installed, or may become affected if the users accepts installation of the control by a malicious web server.

What does the update do? 
The update addresses the vulnerability by correctly validating parameters for the Remote Desktop Web Connection ActiveX control methods.

When this security bulletin was issued, had this vulnerability been publicly disclosed? 
No, 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.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.

*Customers of Microsoft Windows 2000 Service Pack 4 must install KB958471 prior to installing KB958470.

**Administrators may have manually installed this out-of-box download.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.

*Administrators may have manually installed this out-of-box download.

Note For supported versions of Windows XP Professional x64 Edition, this security update is the same as supported versions of the Windows Server 2003 x64 Edition security update.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.

*Administrators may have manually installed this out-of-box download.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.