December 10, 2019
.NET Framework 3.5 and 4.8
The December 10, 2019 update for Windows 10 Version 1903 and Windows Server 1903 RTM and Windows 10, version 1909 and Windows Server, version 1909 includes cumulative reliability improvements in .NET 3.5 and 4.8. We recommend that you apply this update as part of your regular maintenance routines. Before you install this update, see the Prerequisites and Restart requirement sections.
Quality and reliability improvements
ASP.NET now emits a SameSite cookie header when HttpCookie.SameSite value is "None" to accommodate upcoming changes to SameSite cookie handling in Chrome. As part of this change, FormsAuth and SessionState cookies are also issued with SameSite = 'Lax' instead of the previous default of 'None', though these values can be overridden in web.config. Applications that use these cookies across sites – or with iframes – may see a loss of functionality that will require configuration updates to remedy.
For more information, refer to: Work with SameSite cookies in ASP.NET.
Addresses an issue where some Per-Monitor Aware WPF applications that host System-Aware or Unaware child-windows, and run on .NET 4.8, may occasionally encounter a crash with exceptionSystem.Collections.Generic.KeyNotFoundException.
Addresses an issue where some ClickOnce applications or applications creating the default AppDomain with a restricted permission set may observe application launch or application runtime failures, or unexpected behaviors. The observable issue was the System.AppDomainSetup.TargetFrameworkName is null, leading to any quirks enabling reverting back to .NET Framework 4.0 behaviors.
1 Windows Presentation Foundation (WPF)
2 Common Language Runtime (CLR)
Known Issues in this update
There are no known issues with this update, however, this update resolves a previous known issue. This patch includes improvements to resolve a known issue discussed in KB4519573 that occurs after applying the October Preview of Quality Rollup for .NET Framework 4.8. To resolve this issue, install update for .NET Framework 4.8 for Windows 10 Version 1903 and Windows Server 1903 RTM and Windows 10, version 1909 and Windows Server, version 1909 KB4533002.
How to obtain and install the update
Install this update
This update will be downloaded and installed automatically from Windows Update.
To get the standalone package for this update, go to the Microsoft Update Catalog website.
For a list of the files that are provided in this update, download the file information for cumulative update .
To apply this update, you must have .NET Framework 3.5 and 4.8 installed.
You must restart the computer after you apply this update if any affected files are being used. We recommend that you exit all .NET Framework-based applications before you apply this update.