Microsoft Security Bulletin MS12-074 - 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.
• By default, Internet Explorer 9 and Internet Explorer 10 prevent XAML, which is used by XBAPs, from running in the Internet Zone.
• By default, Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8 are configured to prompt the user before running XAML, which is used by XBAPs in the Internet Zone.

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 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. Under Settings, for Loose XAML, click Prompt or Disable, and then click OK.
  3. Click the Security tab, click Internet, and then click Custom level. Under Settings, for XAML browser applications, click Prompt or Disable, and then click OK.
  4. Click the Security tab, click Internet, and then click Custom level. Under Settings, for XPS documents, click Prompt or Disable, and then click OK.
  5. 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.
  6. 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 websites to work incorrectly. If you have difficulty using a website 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 disable XAML browser applications or require a prompt before it runs Loose XAML, XAML browser applications, or XPS documents 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 websites 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, perform the following 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. Perform the following steps:

  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 is an elevation of privilege vulnerability.

What causes the vulnerability? 
The vulnerability is the result of the .NET Framework improperly validating the permissions of objects performing reflection.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability 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.

How could an attacker exploit the vulnerability? 
There are two attack scenarios possible for exploiting this vulnerability: a web browsing scenario and a Windows .NET application bypass of Code Access Security (CAS) restrictions. These scenarios are described as follows:

• Web browsing attack scenario
  An attacker could host a specially crafted website that contains a specially crafted XBAP (XAML browser application) that could exploit this vulnerability and then convince a user to view the website. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements. These websites 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 websites. Instead, an attacker would have to convince users to visit the website, typically by getting them to click a link in an email message or in an Instant Messenger message that takes users to the attacker's website. 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.
• Windows .NET applications attack scenario
  This vulnerability could also be used by Windows .NET Framework applications to bypass Code Access Security (CAS) restrictions.

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 Common Language Runtime Overview.

What is an XAML Browser Application (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 are .NET Framework Code Access Security (CAS) Restrictions? 
The .NET Framework provides a security mechanism called code access security to help protect computer systems from malicious mobile code, to allow code from unknown origins to run with protection, and to help prevent trusted code from intentionally or accidentally compromising security. Code access security (CAS) enables code to be trusted to varying degrees depending on where the code originates and on other aspects of the code's identity. Code access security also enforces the varying levels of trust on code, which minimizes the amount of code that must be fully trusted in order to run. Using code access security can reduce the likelihood that your code will be misused by malicious or error-filled code. For more information on CAS, see the MSDN article, Code Access Security.

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 Windows .NET Framework applications scenario.

• Web browsing scenario
  Successful exploitation of this vulnerability requires that a user is logged on and is visiting websites 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 email on servers. However, best practices strongly discourage allowing this.
• Windows .NET applications
  Workstations and servers that run untrusted Windows .NET Framework applications are also at risk from this vulnerability.

What does the update do? 
The update addresses the vulnerability by correcting how the .NET Framework validates the permissions of objects performing reflection.

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.

Microsoft has not identified any mitigating factors for this vulnerability.

Microsoft has not identified any workarounds for this vulnerability.

What is the scope of the vulnerability? 
This is an information disclosure vulnerability.

What causes the vulnerability? 
The vulnerability is caused when the .NET Framework fails to properly sanitize the output of a function when called from partially trusted code.

What might an attacker use the vulnerability to do? 
In the web browsing scenario, an attacker who successfully exploited this vulnerability could use partially trusted code to obtain confidential information contained on the target system that would only normally be accessible through fully trusted code.

How could an attacker exploit the vulnerability? 
There are two attack scenarios possible for exploiting this vulnerability: a web browsing scenario and a Windows .NET application bypass of Code Access Security (CAS) restrictions. These scenarios are described as follows:

• Web browsing attack scenario
  An attacker could host a specially crafted website that contains a specially crafted XBAP (XAML browser application) that could exploit this vulnerability and then convince a user to view the website. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements. These websites 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 websites. Instead, an attacker would have to convince users to visit the website, typically by getting them to click a link in an email message or in an Instant Messenger message that takes users to the attacker's website. 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.
• Windows .NET applications attack scenario
  This vulnerability could also be used by Windows .NET Framework applications to bypass Code Access Security (CAS) restrictions.

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 Common Language Runtime Overview.

What is an XAML Browser Application (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 are .NET Framework Code Access Security (CAS) Restrictions? 
The .NET Framework provides a security mechanism called code access security to help protect computer systems from malicious mobile code, to allow code from unknown origins to run with protection, and to help prevent trusted code from intentionally or accidentally compromising security. Code access security (CAS) enables code to be trusted to varying degrees depending on where the code originates and on other aspects of the code's identity. Code access security also enforces the varying levels of trust on code, which minimizes the amount of code that must be fully trusted in order to run. Using code access security can reduce the likelihood that your code will be misused by malicious or error-filled code. For more information on CAS, see the MSDN article, Code Access Security.

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 Windows .NET Framework applications scenario.

• Web browsing scenario
  Successful exploitation of this vulnerability requires that a user is logged on and is visiting websites 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 email on servers. However, best practices strongly discourage allowing this.
• Windows .NET applications
  Workstations and servers that run untrusted Windows .NET Framework applications are also at risk from this vulnerability.

What does the update do? 
The update addresses the vulnerability by properly sanitizing output based on the trust level of the calling 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.

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:

• The file sharing protocol, Server Message Block (SMB), is often disabled on the perimeter firewall. This limits the potential attack vectors for this vulnerability.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the logged-on 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.

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 loading of libraries from WebDAV and remote network shares

  Note See Microsoft Knowledge Base Article 2264107 to deploy a workaround tool that allows customers to disable the loading of libraries from remote network or WebDAV shares. This tool can be configured to disallow insecure loading on a per-application or a global system basis.

  Customers who are informed by their vendor of an application being vulnerable can use this tool to help protect against attempts to exploit this issue.

  Note See Microsoft Knowledge Base Article 2264107 to use the automated Microsoft Fix it solution to deploy the registry key to block the loading of libraries for SMB and WebDAV shares. Note that this Fix it solution does require you to install the workaround tool also described in Microsoft Knowledge Base Article 2264107 first. This Fix it solution only deploys the registry key and requires the workaround tool in order to be effective. We recommend that administrators review the KB article closely prior to deploying this Fix it solution.

• Disable the WebClient service

  Disabling the WebClient service helps protect affected systems from attempts to exploit this vulnerability by blocking the most likely remote attack vector through the Web Distributed Authoring and Versioning (WebDAV) client service. After applying this workaround it is still possible for remote attackers who successfully exploit this vulnerability to cause the system to run programs located on the targeted user's computer or the Local Area Network (LAN), but users will be prompted for confirmation before opening arbitrary programs from the Internet.

  To disable the WebClient Service, follow these steps:

  1. Click Start, click Run, type Services.msc and then click OK.
  2. Right-click WebClient service and select Properties.
  3. Change the Startup type to Disabled. If the service is running, click Stop.
  4. Click OK and exit the management application.

  Impact of workaround. When the WebClient service is disabled, Web Distributed Authoring and Versioning (WebDAV) requests are not transmitted. In addition, any services that explicitly depend on the Web Client service will not start, and an error message will be logged in the System log. For example, WebDAV shares will be inaccessible from the client computer.

  How to undo the workaround.

  To re-enable the WebClient Service, follow these steps:

  1. Click Start, click Run, type Services.msc and then click OK.
  2. Right-click WebClient service and select Properties.
  3. Change the Startup type to Automatic. If the service is not running, click Start.
  4. Click OK and exit the management application.
• Block TCP ports 139 and 445 at the firewall

  These ports are used to initiate a connection with the affected component. Blocking TCP ports 139 and 445 at the firewall will help protect systems that are behind that firewall from attempts to exploit this vulnerability. Microsoft recommends that you block all unsolicited inbound communication from the Internet to help prevent attacks that may use other ports. For more information about ports, see the TechNet article, TCP and UDP Port Assignments.

  Impact of workaround. Several Windows services use the affected ports. Blocking connectivity to the ports may cause various applications or services to not function. Some of the applications or services that could be impacted are listed below:

  • Applications that use SMB (CIFS)
  • Applications that use mailslots or named pipes (RPC over SMB)
  • Server (File and Print Sharing)
  • Group Policy
  • Net Logon
  • Distributed File System (DFS)
  • Terminal Server Licensing
  • Print Spooler
  • Computer Browser
  • Remote Procedure Call Locator
  • Fax Service
  • Indexing Service
  • Performance Logs and Alerts
  • Systems Management Server
  • License Logging Service

  How to undo the workaround. Unblock TCP ports 139 and 445 at the firewall. For more information about ports, see TCP and UDP Port Assignments.

What is the scope of the vulnerability? 
This is a remote code execution vulnerability.

What causes the vulnerability? 
The vulnerability is caused when Entity Framework, a .NET Framework component, incorrectly restricts the path used for loading external libraries.

What is the Entity Framework? 
The Entity Framework is a set of technologies in ADO.NET that support the development of data-oriented software applications. For more information, see the MSDN article, Entity Framework Overview.

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. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. If a user is logged on with administrative user rights, an attacker could take complete control of the affected system. 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 convince a user to open a legitimate .NET application built with ADO.NET that is located in the same network directory as a specially crafted dynamic link library (DLL) file. Then, the .NET application could attempt to load the DLL file and execute any code it contained.

What systems are primarily at risk from the vulnerability? 
Workstations and terminal servers are primarily at risk. Servers could be at more risk if administrators allow users to log on to servers and to run programs. However, best practices strongly discourage allowing this.

What does the update do? 
The update addresses the vulnerability by correcting how the .NET Framework loads external libraries.

Is this vulnerability related to Microsoft Security Advisory 2269637? 
Yes, this vulnerability is related to the class of vulnerabilities, described in Microsoft Security Advisory 2269637, that affects how applications load external libraries. This security update addresses a particular instance of this type of vulnerability.

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.

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.

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:

• Configure Internet Explorer not to use WPAD
  1. Click Start, click Control Panel, click Network and Internet, and then click Internet Options.
  2. In the Internet Properties window, click the Connections tab, and then click LAN settings.
  3. In the Local Area Network (LAN) Settings window, uncheck Automatically detect settings and then click OK.

  Impact of workaround. Web proxy auto-discovery will be disabled in Internet Explorer.

  How to undo the workaround. Follow the reverse of the steps above, ensuring that Automatically detect settings is checked in the Local Area Network (LAN) Settings window.

   

• Register WPAD

  Follow the steps provided in Microsoft Knowledgebase Article 934864 to register WPAD.

  Impact of workaround. WPAD will be registered by an administrator before an attacker has the opportunity to do so.

  How to undo the workaround. For more information, see Microsoft Knowledgebase Article 934864.

   

• Do not rely on the default web proxy settings in your .NET application

  Do not rely on the default .NET proxy settings, either explicitly through the WebRequest.DefaultWebProxy property or implicitly, by not setting the proxy (as shown below) in your .NET application. By default, this property is set by the proxy or WPAD settings in Internet Explorer. Developers should explicitly configure the web proxy in the application in the following way:

  IWebProxy proxy = new WebProxy("<IP_ADDRESS>", <PORT_NUMBER>);

  If your .NET application is already using the DefaultWebProxy property, ensure that the name of the desired proxy is hardcoded in the app.config file.

  For documentation on the WebRequest.DefaultWebProxy property, see WebRequest.DefaultWebProxy Property.

  Note This requires recompiling and redeploying affected applications.

What is the scope of the vulnerability? 
This is a remote code execution vulnerability.

What causes the vulnerability? 
The vulnerability is caused by a lack of validation when the .NET Framework acquires the default web proxy settings and executes JavaScript contain within the proxy auto-configuration file.

I'm running Microsoft .NET Framework 4.5 on Windows 8, Windows Server 2012, or Windows RT, am I affected by this issue? 
No, customers running Microsoft .NET Framework 4.5 on Windows 8, Windows Server 2012, or Windows RT are not affected by this issue. The Windows 8 Client and Windows Server 2012 General Availability Cumulative Update (KB2756872) that was released on October 10, 2012 contains additional defense-in-depth changes. Customers who have not already installed this update are encouraged to do so as a defense-in-depth measure. See the More Information section in Microsoft Knowledge Base Article 2745030 for more detail. For download links and further information, see Microsoft Knowledge Base Article 2756872. Note that this update contains non-security related content.

What might an attacker use the vulnerability to do? 
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.

How could an attacker exploit the vulnerability? 
In a man-in-the-middle attack, an attacker can spoof the contents or the location of a proxy auto configuration (PAC) file and then inject code into the currently running application, bypassing the Code Access Security (CAS) restrictions.

If the attacker has access to the local network, the attacker could spoof the location of the PAC file by performing address resolution protocol (ARP) cache poisoning, redirecting requests for the WPAD file to an IP address the attacker controls or by performing NetBIOS Name Service (NBNS) spoofing, where the attacker spoofs responses to WPAD requests, redirecting the target application to a malicious PAC file hosted on a server the attacker owns.

Exploitation could occur if the attacker is able to convince a user to connect to a server outside of the organizational boundary. Specific scenarios that may allow this to occur are as follows:

• DNS devolution, a Windows DNS client feature that allows Windows DNS clients to resolve DNS queries for single-label unqualified hostnames. A malicious user could register a specific host name outside of the organization's boundary that, if clients are configured incorrectly, can be unintentionally contacted by a client when it devolves outside of the organizational boundary while attempting to access that host name.
• DNS spoofing, where an attacker could exploit vulnerabilities in the Windows Domain Name System (DNS), allowing spoofing. These attacks could allow a remote attacker to redirect network traffic intended for systems on the Internet to the attacker’s system.

What are .NET Framework Code Access Security (CAS) Restrictions? 
The .NET Framework provides a security mechanism called Code Access Security to help protect computer systems from malicious mobile code, to allow code from unknown origins to run with protection, and to help prevent trusted code from intentionally or accidentally compromising security. Code Access Security (CAS) enables code to be trusted to varying degrees depending on where the code originates and on other aspects of the code's identity. Code Access Security also enforces the varying levels of trust on code, which minimizes the amount of code that must be fully trusted in order to run. Using Code Access Security can reduce the likelihood that your code will be misused by malicious or error-filled code. For more information on CAS, see the MSDN article, Code Access Security.

What is Web Proxy Auto-Discovery (WPAD)? 
The Web Proxy Auto-Discovery (WPAD) feature enables web clients to automatically detect proxy settings without user intervention. The WPAD feature prepends the hostname "wpad" to the fully-qualified domain name and progressively removes subdomains until it finds a WPAD server answering the domain name. For more information see WinHTTP AutoProxy Support.

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 Windows .NET Framework applications scenario.

• Web browsing scenario
  Successful exploitation of this vulnerability requires that a user is logged on and is visiting websites 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 email on servers. However, best practices strongly discourage allowing this.
• Windows .NET applications scenario
  Workstations and server that run untrusted Windows .NET Framework applications are also at risk from this vulnerability.

What does the update do? 
The update addresses the vulnerability by updating Microsoft.Jscript within System.Net and restricting how the .NET Framework executes JavaScript contained within a WPAD configuration file. The update also changes the way WPAD is handled by using WinHTTP instead of Microsoft.Jscript. This change, while unlikely, may cause some compatibility issues for environments that have non-standard proxy auto-configuration environments. For more information about this change, see the More Information section in Microsoft Knowledge Base Article 2745030.

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.

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.
• By default, Internet Explorer 9 and Internet Explorer 10 prevent XAML, which is used by XBAPs, from running in the Internet Zone.
• By default, Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8 are configured to prompt the user before running XAML, which is used by XBAPs in the Internet Zone.

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 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. Under Settings, for Loose XAML, click Prompt or Disable, and then click OK.
  3. Click the Security tab, click Internet, and then click Custom level. Under Settings, for XAML browser applications, click Prompt or Disable, and then click OK.
  4. Click the Security tab, click Internet, and then click Custom level. Under Settings, for XPS documents, click Prompt or Disable, and then click OK.
  5. 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.
  6. 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 websites to work incorrectly. If you have difficulty using a website 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 disable XAML browser applications or require a prompt before it runs Loose XAML, XAML browser applications, or XPS documents 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 websites 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, perform the following 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. Perform the following steps:

  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 is an elevation of privilege vulnerability.

What causes the vulnerability? 
The vulnerability is the result of the .Net Framework improperly validating permissions of objects involved with reflection.

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.

How could an attacker exploit the vulnerability? 
There are two attack scenarios possible for exploiting this vulnerability: a web browsing scenario and a Windows .NET application bypass of Code Access Security (CAS) restrictions. These scenarios are described as follows:

• Web browsing attack scenario
  An attacker could host a specially crafted website that contains a specially crafted XBAP (XAML browser application) that could exploit this vulnerability and then convince a user to view the website. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements. These websites 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 websites. Instead, an attacker would have to convince users to visit the website, typically by getting them to click a link in an email message or in an Instant Messenger message that takes users to the attacker's website. 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.
• Windows .NET applications attack scenario
  This vulnerability could also be used by Windows .NET Framework applications to bypass Code Access Security (CAS) restrictions.

What is the WPF? 
The Microsoft .NET Framework provides a run-time environment called the Windows Presentation Foundation (WPF), which is a presentation system for building Windows client applications. For more information about WPF, see Introduction to WPF.

What is an XAML Browser Application (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 are .NET Framework Code Access Security (CAS) Restrictions? 
The .NET Framework provides a security mechanism called code access security to help protect computer systems from malicious mobile code, to allow code from unknown origins to run with protection, and to help prevent trusted code from intentionally or accidentally compromising security. Code access security (CAS) enables code to be trusted to varying degrees depending on where the code originates and on other aspects of the code's identity. Code access security also enforces the varying levels of trust on code, which minimizes the amount of code that must be fully trusted in order to run. Using code access security can reduce the likelihood that your code will be misused by malicious or error-filled code. For more information on CAS, see the MSDN article, Code Access Security.

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 Windows .NET Framework applications scenario.

• Web browsing scenario
  Successful exploitation of this vulnerability requires that a user is logged on and is visiting websites 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 email on servers. However, best practices strongly discourage allowing this.
• Windows .NET applications
  Workstations and servers that run untrusted Windows .NET Framework applications are also at risk from this vulnerability.

What does the update do? 
The update addresses the vulnerability by correcting how the .NET Framework optimizes code in memory.

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 The update for supported versions of Windows XP Professional x64 Edition also applies to supported versions of Windows Server 2003 x64 Edition.

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 The update for supported versions of Windows Server 2003 x64 Edition also applies to supported versions of Windows XP Professional x64 Edition.

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.

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.