February 9, 2021-KB4601051 Cumulative Update for .NET Framework 4.8 for Windows 10, version 1607 and Windows Server, version 2016

2021-02-0908:00:00

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February 9, 2021-KB4601051 Cumulative Update for .NET Framework 4.8 for Windows 10, version 1607 and Windows Server, version 2016

Release Date:
February 9, 2021Version: .NET Framework 4.8

Summary

Security ImprovementsThis security update addresses a denial of service vulnerability in .NET Framework. For more information please see CVE-2021-24111.Quality ImprovementsASP.NET	- Addresses an issue where after installing the update released on October 20th, some ASP.Net applications fail during precompilation – likely with a message that contains the words “Error ASPCONFIG.”
WPF1	- Addresses a hang when scrolling to the end of a TreeView, when layout rounding is enabled and DPI scaling is not 100%.
CLR	- Improves the reliability of automatic Native Image generation task.
1 Windows Presentation Foundation (WPF)

Known issues in this update

Symptom| After installing this update, WPF apps may crash with a callstack similar to`

Exception Info: System.NullReferenceException at System.Windows.Interop.HwndMouseInputProvider.HasCustomChrome(System.Windows.Interop.HwndSource, RECT ByRef) at System.Windows.Interop.HwndMouseInputProvider.GetEffectiveClientRect(IntPtr)
at System.Windows.Interop.HwndMouseInputProvider.PossiblyDeactivate(IntPtr, Boolean)
at System.Windows.Interop.HwndMouseInputProvider.Dispose()

This occurs when disposing an HwndSource whose RootVisual is null, a situation that arises in Visual Studio when docking or splitting windows, and could arise in other apps. ---|--- **Workaround**| To work around this problem, set two AppContext switches using one of the methods described in [AppContext Class (System)](<https://docs.microsoft.com/en-us/dotnet/api/system.appcontext?view=netcore-3.1#remarks>) under the heading “AppContext for library consumers”. The switches are named**Switch.System.Windows.Interop.MouseInput.OptOutOfMoveToChromedWindowFix**and**Switch.System.Windows.Interop.MouseInput.DoNotOptOutOfMoveToChromedWindowFix** and both should be set to “true”. The first switch avoids the crash, but re-introduces the bug fixed in the KBs. The second switch is currently ignored, but will be recognized in a future .NET update that contains a fix for the null-reference crash; it restores the original bug fix.For example, using the app.config file method to apply the workaround at application scope:

How to get this update

Install this updateRelease Channel	Available	Next Step
Windows Update and Microsoft Update	Yes	None. This update will be downloaded and installed automatically from Windows Update.
Microsoft Update Catalog	Yes	To get the standalone package for this update, go to the Microsoft Update Catalog website.
Windows Server Update Services (WSUS)	Yes	This update will automatically sync with WSUS if you configure Products and Classifications as follows:Product: Windows 10, version 1607 and Windows Server, version 2016Classification: Security Updates File informationFor a list of the files that are provided in this update, download the file information for cumulative update.