Description of the Security Only update for the .NET Framework 4.6 and 4.6.1 for Windows 8.1 and Windows Server 2012 R2: May 9, 2017

Summary

This security update for the Microsoft .NET Framework resolves a security feature bypass vulnerability in which the .NET Framework (and the .NET Core) components don't completely validate certificates. To learn more about this vulnerability, see Microsoft Common Vulnerabilities and Exposures CVE-2017-0248.

This update also contains security-enhancing fixes to the Windows Presentation Framework PackageDigitalSignatureManager component's ability to sign packages with the SHA256 hash algorithm.

Important

• All future security and nonsecurity updates for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 will require update 2919355 to be installed. We recommend that you install update 2919355 on your Windows RT 8.1-based, Windows 8.1-based, or Windows Server 2012 R2-based computer so that you receive future updates.

• If you install a language pack after you install this update, you must reinstall this update. Therefore, we recommend that you install any language packs that you need before you install this update. For more information, see Add language packs to Windows.

Additional information about this security update

• Enhanced Key Usage (EKU) is described in RFC 5280 in section 4.2.1.12: This extension indicates one or more purposes for which the certified public key may be used, in addition to or in place of the basic purposes that are indicated in the key usage extension. For example, a certificate that’s used for the authentication of a client to a server must be configured for Client Authentication. Similarly, a certificate that’s used for the authentication of a server must be configured for Server Authentication.
  
  When certificates are used for authentication, the authenticator examines the client certificate and seeks the correct purpose object identifier in Application Policies extensions. For example, the object identifier for Client Authentication is 1.3.6.1.5.5.7.3.2. When a certificate is used for client authentication, this object identifier must be present in the EKU extensions of the certificate, or authentication fails. Certificates that have no EKU extension continue to authenticate correctly.
  
  If you temporarily can’t access correctly reissued certificates, you can choose to opt in or out of the security change across all computer operations to avoid any connectivity effects. To do this, specify the following registry key settings, depending on which .NET Framework version your application is targeting.
  
  Method 1: Updating registry key (available for all versions)
  
  Note This registry entry must be a DWORD entry.
  

  • For 32-bit process on 32-bit system and 64-bit process on 64-bit system:
    

    HKEY_LOCAL_MACHINE\Software\Microsoft\.NETFramework\v4.0.30319@RequireCertificateEKUs=0

  • For 32-bit process on 64-bit system:
    

    HKEY_LOCAL_MACHINE \Software\Wow6432Node\Microsoft\.NETFramework\v4.0.30319@RequireCertificateEKUs=0

  You can also opt out on a per-application basis. The following options are available for disabling this change to make sure that application compatibility is maintained.
  
  Method 2: Disable the policy for individual applications
  
  Note This registry entry must be a DWORD entry. The only valid value is 0. Any other value is ignored.

  • For 32-bit process on 32-bit system and 64-bit process on 64-bit system:

    HKEY_LOCAL_MACHINE\Software\Microsoft\.NETFramework\v4.0.30319@System.Net.ServicePointManager.RequireCertificateEKUs
    S:\Prj\console_pg\console_pg45\bin\Release\console_pg45.exe=0
    C:\MyApp\MyApp.exe=0

  • For 32-bit process on 64-bit system:

    HKEY_LOCAL_MACHINE \Software\Wow6432Node\Microsoft\.NETFramework\v4.0.30319@System.Net.ServicePointManager.RequireCertificateEKUs
    S:\Prj\console_pg\console_pg45\bin\Release\console_pg45.exe=0
    C:\MyApp\MyApp.exe=0

  Method 3: Using configuration API (available for the .NET Framework 4.6 and later versions)
  
  Starting in the .NET Framework 4.6, you can change the configuration at an application level through code, an application configuration, or registry changes.
  
  Configuring the switch in the .NET Framework 4.6
  
  Note The following examples disable the security feature.

  • Programmatically
    
    The first thing that the application should do is to run the following code. This is because Service Point Manager initializes only one time.
      private const string DisableCachingName = @"TestSwitch.LocalAppContext.DisableCaching"; private const string DontCheckCertificateEKUsName= @"Switch.System.Net.DontCheckCertificateEKUs"; AppContext.SetSwitch(DisableCachingName, true); AppContext.SetSwitch(DontCheckCertificateEKUsName, true);

  • Application configuration
    
    To change the application configuration, add the following entry:
      <runtime> <AppContextSwitchOverrides value="Switch.System.Net.DontCheckCertificateEKUsName=true"/> </runtime>

  • Registry key (machine global):
    

    Registry location: HKEY_LOCAL_MACHINE\Software\[Wow6432Node\]Microsoft\.NETFramework\AppContext\Switch.System.Net.DontCheckCertificateEKUsName

    Type: String
    Value: "true"

  Note By default, Switch.System.Net.DontCheckCertificateEKUsName = True for all .NET Framework 4.x applications that are running on the .NET Framework 4.6 and later versions.

• For more information about this security update as it relates to Windows 8.1 and Windows Server 2012 R2, see the following article in the Microsoft Knowledge Base:

  4019111 Security Only update for the .NET Framework 3.5 Service Pack 1, 4.5.2, 4.6, 4.6.1, and 4.6.2 updates for Windows 8.1 and Windows Server 2012 R2: May 9, 2017

How to obtain and install the update

File information

How to obtain help and support for this security update

• Help for installing updates: Windows Update FAQ

• Security solutions for IT professionals: TechNet Security Support and Troubleshooting

• Help for protecting your Windows-based products and services from viruses and malware: Microsoft Secure

• Local support according to your country: International Support

Applies to

This article applies to the following:
 

• Microsoft .NET Framework 4.6 and 4.6.1 when used with:

  • Windows Server 2012 R2

  • Windows 8.1