Microsoft Security Bulletin MS10-051 - 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:

• 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 convince 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.
• By default, all supported versions of Microsoft Outlook, Microsoft Outlook Express, and Windows Mail open HTML e-mail messages in the Restricted sites zone, which disables script and ActiveX controls, removing the risk of an attacker being able to use this vulnerability to execute malicious code. 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.

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 COM objects 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. The COM object that will be prevented from instantiating in Internet Explorer by this workaround is the Msxml2.XMLHTTP.3.0 ActiveX control.

  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 a control from running in Internet Explorer, see Microsoft Knowledge Base Article 240797. Follow the steps in this article to create a Compatibility Flags value in the registry to prevent a COM object from being instantiated in Internet Explorer.

  To set the kill bit for the CLSID with the value of {F5078F35-C551-11D3-89B9-0000F81FE221}, paste the following text in a text editor such as Notepad. Then, save the file by using the .reg file name extension.

  Windows Registry Editor Version 5.00
  [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{F5078F35-C551-11D3-89B9-0000F81FE221}]
  "Compatibility Flags"=dword:00000400

  You can 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. There is no impact as long as the COM object is not intended to be used in Internet Explorer.

  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 block 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 block 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 blocking 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. Blocking ActiveX Controls or Active Scripting is a global setting that affects all Internet and intranet sites. If you do not want to block ActiveX Controls or Active Scripting for such 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 block 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 yourself 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.

• Configure Internet Explorer to prompt before running Active Scripting or to disable Active Scripting in the Internet and Local intranet security zone

  You can help protect against exploitation of this vulnerability by changing your settings to prompt before running Active Scripting or to disable Active Scripting in the Internet and Local intranet security zone. To do this, follow these steps:

  1. In Internet Explorer, click Internet Options on the Tools menu.
  2. Click the Security tab.
  3. Click Internet, and then click Custom Level.
  4. Under Settings, in the Scripting section, under Active Scripting, click Prompt or Disable, and then click OK.
  5. Click Local intranet, and then click Custom Level.
  6. Under Settings, in the Scripting section, under Active Scripting, click Prompt or Disable, and then click OK.
  7. Click OK two times to return to Internet Explorer.

  Note Disabling Active Scripting in the Internet and Local intranet security zones 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.

  Impact of workaround. There are side effects to prompting before running Active Scripting. Many Web sites that are on the Internet or on an intranet use Active Scripting to provide additional functionality. For example, an online e-commerce site or banking site may use Active Scripting to provide menus, ordering forms, or even account statements. Prompting before running 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 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.

• Restrict Web sites to only your trusted Web sites

  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 Internet Explorer's 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.

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

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. 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.

What causes the vulnerability? 
There exists a bug in the way MSXML handles HTTP responses which may corrupt the system state in such a way that an attacker could run arbitrary code.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability could gain the same user rights 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 exploit the vulnerability by hosting a specially crafted Web site that is designed to invoke MSXML through Internet Explorer. This can also include compromised Web sites and Web sites that accept or host user-provided content or advertisements. 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 convince users to visit the Web site, typically by getting them to click a link in 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 is logged on and visits a Web site for any malicious action to occur. Therefore, any systems where Internet Explorer is used frequently, such as workstations or terminal servers, are at the most risk from this vulnerability.

I am running Internet Explorer for Windows Server 2003 or Windows Server 2008. Does this mitigate this vulnerability? 
Yes. By default, Internet Explorer on Windows Server 2003 and Windows Server 2008 runs in a restricted mode that is known as Enhanced Security Configuration. Enhanced Security Configuration is a group of preconfigured settings in Internet Explorer that can reduce the likelihood of a user or administrator downloading and running specially crafted Web content on a server. This is a mitigating factor for Web sites that you have not added to the Internet Explorer Trusted sites zone. See also Managing Internet Explorer Enhanced Security Configuration.

What does the update do? 
The update removes the vulnerability by ensuring that HTTP responses are handled correctly by MSXML.

When this security bulletin was issued, had this vulnerability been publicly disclosed? 
No. Microsoft received information about this vulnerability through coordinated vulnerability disclosure.

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.

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.

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.

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.