Microsoft Security Bulletin MS13-004 - Important

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 the result of the .NET Framework improperly initializing the contents of a memory array.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability could bypass Code Access Security (CAS) restrictions and obtain data that is stored in unmanaged memory locations. This data could contain sensitive information that is not otherwise accessible. Note that this vulnerability would not allow an attacker to execute code or to elevate their user rights directly.

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 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 and obtain the information stored in the unmanaged memory locations.

What is Windows Forms (WinForms)? 
Windows Forms is a smart client technology for the .NET Framework, a set of managed libraries that simplify common application tasks such as reading and writing to the file system. When you use a development environment like Visual Studio, you can create Windows Forms smart-client applications that display information, request input from users, and communicate with remote computers over a network. For more information, see Windows Forms 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 initializes memory arrays.

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:

• Firewall best practices and standard default firewall configurations can help protect networks from attacks that originate outside the enterprise perimeter. Best practices recommend that systems that are connected to the Internet have a minimal number of ports exposed.
• 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.
• 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 number of objects in memory prior to copying those objects into an array.

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 Windows Forms (WinForms)? 
Windows Forms is a smart client technology for the .NET Framework, a set of managed libraries that simplify common application tasks such as reading and writing to the file system. When you use a development environment like Visual Studio, you can create Windows Forms smart-client applications that display information, request input from users, and communicate with remote computers over a network. For more information, see Windows Forms 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 copies objects 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.

• Firewall best practices and standard default firewall configurations can help protect networks from attacks that originate outside the enterprise perimeter. Best practices recommend that systems that are connected to the Internet have a minimal number of ports exposed.
• 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.
• 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 size of objects in memory prior to copying those objects into an array.

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 System.DirectoryServices.Protocols (S.DS.P)? 
System.DirectoryServices.Protocols (S.DS.P) is a namespace that brings Lightweight Directory Access Protocol (LDAP) programming to managed code developers. For more information about S.DS.P, see Introduction to System.DirectoryServices.Protocols (S.DS.P).

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 size of an array prior to copying objects 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.

• Firewall best practices and standard default firewall configurations can help protect networks from attacks that originate outside the enterprise perimeter. Best practices recommend that systems that are connected to the Internet have a minimal number of ports exposed.
• 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.
• 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 object.

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.

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.