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.
- 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
Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
- Enhanced Key Usage (EKU) is described in RFC 5280 in section 22.214.171.124: 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 126.96.36.199.188.8.131.52.2. 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.
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.
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.
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 32-bit process on 32-bit system and 64-bit process on 64-bit systems:
- For 32-bit process on 64-bit system:
- 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
- For 32-bit process on 64-bit system: HKEY_LOCAL_MACHINE \Software\Wow6432Node\Microsoft\.NETFramework\v4.0.30319@System.Net.ServicePointManager.RequireCertificateEKUs
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:
- Registry key (machine global):
Registry location: HKEY_LOCAL_MACHINE\Software\[Wow6432Node\]Microsoft\.NETFramework\AppContext\Switch.System.Net.DontCheckCertificateEKUsName
For more information about this security update as it relates to Windows Server 2012, see the following article in the Microsoft Knowledge Base:
4019110 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 Server 2012: May 9, 2017
How to obtain and install the update
Method 1: Microsoft Update Catalog
To get the stand-alone package for this update, go to the Microsoft Update Catalog.
Method 2: Windows Software Update Services (WSUS)
On your WSUS server, follow these steps:
- Click Start, click Administrative Tools, and then click Microsoft Windows Server Update Services 3.0.
- Expand ComputerName, and then click Action.
- Click Import Updates.
- WSUS will open a browser window in which you may be prompted to install an ActiveX control. You must install the ActiveX control to continue.
- Once the control is installed, you will see the Microsoft Update Catalog screen. Enter 4019110 in the Search box, and then click Search.
- Locate the .NET Framework packages that match the operating systems, languages, and processors in your environment. Click Add to add them to your basket.
- When you have selected all the packages you need, click View Basket.
- Click Import to import the packages to your WSUS server.
- Click Close once the packages have been imported to return to WSUS.
The updates are now available for installation through WSUS.
Update deployment information
For deployment details for this security update, see the following article in the Microsoft Knowledge Base:
20170509 Security update deployment information: May 9, 2017
Update removal information
Note We do not recommend that you remove any security update.
To remove this update, use the Programs and Features item in Control Panel.
Update restart information
This update does not require a system restart after you apply it unless files that are being updated are locked or are being used.
Update replacement information
This update does not replace any previously released update.
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