Microsoft: Patch Tuesday broke .NET on Windows 11/10, these OOB updates resolved the issues

Microsoft released Patch Tuesday updates on Windows 10 (KB5027215, among others), and Windows 11 (KB5027231) on June 13, which was the second Tuesday of the month. The update addressed security issues, among other bugs. Aside from OS security, Patch Tuesday also fixed security issues in Office 2013 and 2016, for both 32-bit and 64-bit versions. The company also announced the arrival of a full-screen notice about Windows Hello that will now be displayed on both Windows 11 as well as Windows 10.

And as is often the case, there are major bugs affecting it as well. On Windows 11, Patch Tuesday was causing Malwarebytes to go a bit haywire and block Google Chrome. Meanwhile, users reported that Windows 10 updates were having installation issues.

Microsoft has now also patched an issue related to how the .NET Framework runtime update on June 13 would affect the imports of X.509 Certificates. Unlike how they functioned before, there the additional validation could now lead to CryptographicException error.

Description of change

Prior to the June 13, 2023, change, when .NET Framework and .NET is presented with a binary certificate blob for import, .NET Framework and .NET would typically delegate validation and import of the blob to the underlying OS. For example, on Windows, .NET Framework and .NET would typically rely on the PFXImportCertStore API for validation and import.

As of the June 13, 2023, change, when .NET Framework and .NET is presented with a binary certificate blob for import, .NET Framework and .NET will in some circumstances perform additional validation before handing the blob to the underlying OS. This additional validation performs a series of heuristic checks to determine if the incoming certificate would maliciously exhaust resources upon import. Since this is additional validation beyond what the underlying OS would normally perform, it may block certificate blobs which would have successfully imported prior to the June 13, 2023, change.

Microsoft has also detailed the symptoms of the issue:

Symptom

When using the X509Certificate, X509Certificate2, or X509Certificate2Collection class to import a PKCS#12 blob containing a private key, the calling application may observe the below exception.

System.Security.Cryptography.CryptographicException: PKCS12 (PFX) without a supplied password has exceeded maximum allowed iterations.

This failure affects PKCS#12 blobs which have been exported [e.g., via X509Certificate.Export(X509ContentType.Pfx)] without a password. The failure may occur non-deterministically.

A workaround for the problem had been deployed on affected systems, though the company states that any registry changes made to work around the problem must be reverted:

Workaround

Microsoft has released updated installers for .NET Framework and .NET to address this issue. These installers can be applied to the affected machine regardless of whether the machine has already applied the original June 13, 2023, .NET Framework and .NET security updates.

Important:

If you previously used the registry switches documented at KB5025823 Change in how .NET applications import X.509 certificates to work around this issue, please remove those registry switches before installing the new patch. Run the two commands below from an elevated command prompt to remove the registry switches.

reg delete "HKLM\Software\Microsoft\.NETFramework" /v Pkcs12UnspecifiedPasswordIterationLimit /reg:32

reg delete "HKLM\Software\Microsoft\.NETFramework" /v Pkcs12UnspecifiedPasswordIterationLimit /reg:64

These issues are addressed on Windows 10 as well as Windows 11, and more with the following out-of-band updates that can be manually downloaded from the Microsoft Update Catalog website.

You may find more details about the issue on Microsoft's official website on this page (KB5028608).

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