Microsoft Security Bulletin MS10-077 - 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:

• By default, Internet Explorer on Windows Server 2003, Windows Server 2008, and Windows Server 2008 R2 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability only on Windows Server 2008 and Windows Server 2008 R2, and only in a Web-based attack scenario. See the FAQ section of this vulnerability 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 or the user account of ASP.NET. Users or accounts that are configured to have fewer user rights on the system could be less impacted than users or accounts that 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 convince users to visit the Web site, typically by getting them to click a link in an e-mail or Instant Messenger message that takes users to the attacker's Web site.
• In a Web-hosting scenario, an attacker must have permission to upload arbitrary ASP.NET pages to a Web site and ASP.NET must be installed on that Web server. In default configuration, an anonymous user cannot upload and run Microsoft .NET code on an Internet Information Server (IIS).
• Microsoft Silverlight is not affected by this vulnerability.
• 32-bit Microsoft Windows editions are not affected by this vulnerability.

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:

• Disable partially trusted Microsoft .NET applications

  To disable all Microsoft .NET applications running at partial trust, including XAML browser applications (XBAPs) and Microsoft .NET applications located on the network, run the following commands from an elevated command prompt:

  caspol –pp off
caspol –m –resetlockdown
caspol –pp on

  Note You must be logged in as administrator or have administrative credentials to complete this workaround.

  Impact of workaround. Some Microsoft .NET applications will not run.

  How to undo the workaround.

  To reset the Microsoft .NET security policies to the defaults, run the following commands from an elevated command prompt:

  caspol –pp off
caspol –m –reset
caspol –pp on

  Note You must be logged in as administrator or have administrative credentials to undo this workaround.

• Disable XAML browser applications in Internet Explorer

  To help protect against this vulnerability, change your settings to prompt before running XAML browser applications (XBAPs) or to disable XBAPs in the Internet and Local intranet security zones as follows:

  1. In Internet Explorer, click the Tools menu and then select Internet Options.
  2. Click the Security tab, click Internet, and then click Custom level.
  3. Under Settings, for Loose XAML, click Prompt or Disable, and then click OK. Repeat steps 2 and 3 for XAML browser applications and XPS documents.
  4. On the Security tab, click Custom level. Under .NET Framework-reliant components, set Run components not signed with Authenticode to either Prompt or Disable, and then click OK. Repeat this step for Run components signed with Authenticode, and then click OK.
  5. Click Local intranet, and then click Custom Level. Repeat steps 3 and 4. If you are prompted to confirm that you want to change these settings, click Yes. Click OK to return to Internet Explorer.

  Impact of workaround. Microsoft .NET code will not run in Internet Explorer or will not run without prompting. Disabling Microsoft .NET applications and components 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.

  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.

  How to undo the workaround.

  1. In Internet Explorer, click the Tools menu, and then select Internet Options.
  2. Click the Security tab, click Reset all zones to default level, and then click OK.

What is the scope of the vulnerability? 
This vulnerability is a remote code execution vulnerability in either the context of the currently logged-on user, or in the context of the service account associated with an application pool identity.

What causes the vulnerability? 
This vulnerability results from the manner in which the JIT compiler optimizes code based on an incorrect assessment, which can lead to memory corruption and, as a result, execution of arbitrary, unmanaged code.

What is the .NET Framework JIT compiler? 
The .NET Framework JIT compiler is a .NET Framework runtime component which compiles Microsoft Intermediate Language (MSIL) against the Common Language Runtime (CLR) on demand at application run time, when the contents of an assembly are loaded and executed.

What is the CLR? 
The Microsoft .NET Framework provides a run-time environment called the Common Language Runtime (CLR), which runs the code and provides services that make the development process easier. For more information about the CLR, see the MSDN article, Common Language Runtime Overview.

What is the MSIL? 
The Microsoft Intermediate Language (MSIL) is a CPU-independent instruction set that is generated by the Microsoft .NET Framework compilers and consumed by the .NET Common Language Runtime (CLR). Before MSIL can be executed, it must be converted to native, CPU-specific code by the .NET CLR.

What might an attacker use the vulnerability to do? 
In the Web browsing scenario, an attacker who successfully exploited this vulnerability could obtain the same permissions as the currently 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.

In the Web hosting scenario, an attacker who successfully exploits this vulnerability could obtain the same permissions as the service account associated with the application pool identity of the application pool in which a Microsoft .NET application is running. Depending on application pool isolation configuration and permissions granted to the service account, an attacker might be able to take control over other application pools on the Web server or be able to take complete control of the affected system. For more information about application pool identities and configuration, see the TechNet article, Configure Application Pool Identity.

How could an attacker exploit the vulnerability? 
There are two scenarios possible for exploitation of this vulnerability: a Web browsing scenario and a Web hosting scenario. These scenarios are described as follows:

• Web browsing scenario
  An attacker could host a specially crafted Web site that contains a specially crafted XBAP (XAML browser application) that could exploit this vulnerability and then convince a user to view the Web site. The attacker could also take advantage of 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 message 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.
• Web hosting scenario
  If a Web hosting environment allows users to upload custom ASP.NET applications, an attacker could upload a malicious ASP.NET application that uses this vulnerability to break out of the sandbox used to prevent ASP.NET code from performing harmful actions on the server system.

What is an XBAP? 
An XAML browser application (XBAP) combines features of both a Web application and a rich-client application. Like Web applications, XBAPs can be published to a Web server and launched from Internet Explorer. Like rich-client applications, XBAPs can take advantage of the capabilities of Windows Presentation Foundation (WPF). For more information about XBAPs, see MSDN article, Windows Presentation Foundation XAML Browser Applications Overview.

What systems are primarily at risk from the vulnerability? 
There are two types of systems at risk from this vulnerability, described as follows: systems that are using the Web browsing scenario, and systems that are using the Web hosting scenario.

• Web browsing scenario
  Successful exploitation of this vulnerability requires that a user is logged on and is visiting Web sites using a Web browser capable of instantiating XBAPs. Therefore, any systems where a Web browser is used frequently, such as workstations or terminal servers, are at the most risk from this vulnerability. Servers could be at more risk if administrators allow users to browse and read e-mail on servers. However, best practices strongly discourage allowing this.
• Web hosting scenario
  Web hosting sites that allow users to upload custom ASP.NET applications are at increased risk.

I am running Internet Explorer for Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2. Does this mitigate this vulnerability? 
Yes and no. For the Web browsing attack scenario, running Internet Explorer on Windows Server 2008 or Windows Server 2008 R2 mitigates the vulnerability, but running Internet Explorer on Windows Server 2003 does not mitigate the vulnerability. Also, for the Web hosting attack scenario, none of these platforms mitigate the vulnerability. 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 from a server. This is a mitigating factor for Web sites that you have not added to the Internet Explorer Trusted sites zone, but it applies to Windows Server 2008 and Windows Server 2008 R2 only. See also Managing Internet Explorer Enhanced Security Configuration.

What does the update do? 
This update addresses the vulnerability by modifying the way in which the JIT compiler optimizes code.

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 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.