Windows 11: Security update blocks computers

Microsoft's latest security update is causing chaos for Windows 11 users. The September update, intended to fix vulnerabilities, instead causes installation errors and destroys network file sharing. Both private users and businesses are affected – by updates that were supposed to protect their systems.

This forces millions of users into a dilemma: remain unprotected or risk the system no longer functioning?

Installation fails with mysterious error codes

Shortly after the release of the September cumulative update KB5065426 for Windows 11 version 24H2, reports of failed installations are increasing. Users are receiving various error codes such as 0x800F0991, 0x800F0922, and 0x800F081F – the update simply cannot be installed.

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Even proven repair tools like the System File Checker (SFC) or DISM commands don't provide a solution. What's particularly frustrating is that even manually downloading the update from the Microsoft Update Catalog doesn't help. The installation simply aborts at a certain percentage with the blunt message, "The update was not installed."

The problem is that these are mandatory security patches. Failed installations leave systems unprotected—a security risk for users and companies.

Microsoft confirms: File sharing destroyed

At the same time, Microsoft officially acknowledged a second issue: The September update breaks connections to SMBv1 file shares. After installation, users can no longer access shared folders and files that use the older protocol over NetBIOS.

All current Windows versions are affected: Windows 11 (24H2, 23H2, 22H2), Windows 10, and Windows Server. The issue occurs if either the accessing computer or the server has the September Update installed.

As a workaround, Microsoft offers to open TCP port 445. This forces SMB connections to bypass NetBT and run directly over TCP. A temporary solution, but it demonstrates the extent of the update glitch.

The SMBv1 legacy: Old, uncertain, but still there

The Server Message Block protocol is the standard for file sharing on networks. However, SMBv1 is considered hopelessly outdated and insecure. Microsoft declared it deprecated in 2014 and has not installed it by default since Windows 10 version 1709.

Why? SMBv1 vulnerabilities enabled devastating ransomware attacks like WannaCry and NotPetya in 2017. These attacks used NSA exploits against SMBv1 to spread worldwide and cause billions in damage.

Despite all the warnings, some companies still rely on SMBv1 – whether because of old hardware or outdated software. For them, the current update poses a double burden: insecure and now unusable.

Fixes in sight – but when?

Microsoft promises a permanent fix for the SMBv1 issue by the end of September or early October. There is currently no timeline or official statement regarding the widespread installation errors.

Affected users are in a bind: either remaining unpatched and exposing themselves to security risks, or installing updates and risking system problems. This week's events demonstrate how complex IT security has become – when the cures themselves become the disease.

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Microsoft recommends users report issues via the Windows Feedback Hub. Until a solution is found, the only option is to keep an eye on official support channels and hope that the next updates work.