Microsoft update KB4345421 totally breaks .NET Framework, no workaround available

Microsoft just recently confirmed in a blog post that the cumulative update KB4345421 has a botched .NET Framework patch, which breaks .NET Framework and applications that rely on it – this effects both Windows 10 PC and Windows 10 devices.

Even worse, all of the July 16 cumulative updates are affected, including KB4345421 for Windows 10 April 2018 Update (version 1803), KB4345420 for Fall Creators Update (version 1709), and KB4345419 for Creators Update (version 1703).

After installing one of these updates, .NET Framework applications will fail to load, spewing out error messages including read access denied, class not registered, or internal failure occurred for unknown reasons. Microsoft has said the most common failure signature is “Exception type: System.UnauthorizedAccessException. Message: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)).”

There is currently no workaround as of this writing, and Microsoft has said that users can uninstall the updates to restore functionality. In an advisory, Microsoft said:

“Applications that rely on .NET Framework to initialize a COM component and that run with restricted permissions may fail to start or run correctly after you install the July 2018 Security and Quality Rollup updates for .NET Framework.

The .NET Framework runtime uses the process token to determine whether the process is being run within an elevated context. These system calls can fail if the required process inspection permissions are not present. This causes an “access denied” error.”

The July 16 updates that have this .NET Framework problem were originally released to address issues in the original July 2018 updates, but it looks like Microsoft is going to need to release a third round of updates to correct this new .NET Framework issue.

No information is available on when Microsoft will release updates to fix the .NET Framework issue, but hopefully it will be soon.

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