Microsoft’s September 2026 security update for Windows 11, KB5124008, has started breaking things for some users. This comes just days after the company urged everyone to install it quickly. The update, released on September 8 for builds 26100.9445 (24H2) and 26200.9445 (25H2), patched two actively exploited zero-day flaws. Microsoft’s own support page now lists three confirmed known issues introduced by the same patch.
What KB5124008 is breaking
Microsoft’s support documentation confirms three separate problems tied to this update:
- USB Audio Class 1.0 devices — some USB audio hardware fails to start, showing a “This device cannot start (Code 10)” error in Device Manager. Affected users get no sound output, unresponsive volume controls, or lose multichannel and 3D audio modes while stereo output still works.
- Hyper-V shared folders for Linux VMs — apps that use HCS-managed virtual machines with Plan9 folder sharing, including WSL and Claude Cowork, can no longer see or access folders shared from the Windows host. Standard Hyper-V VMs are not affected.
- Remote Desktop Services instability — some organizations are seeing RDP connections fail after a few minutes, sign-in issues, servers stuck at the “Please wait for the Remote Desktop Configuration” screen, and an unresponsive Windows Update page.
Beyond Microsoft’s official list, users and outlets including Windows Latest and Neowin have reported more problems this week. These have not yet been added to Microsoft’s known-issues page. They include Explorer.exe crashing to a black screen after sign-in. Others report File History failing to detect external drives, and driver timeouts on some AMD Radeon GPUs. A domain-logon failure affecting Netlogon secure channel negotiation with older Windows Server domain controllers has also surfaced. So has a report of broken Always On VPN connections using certificate-based authentication. Both are being tracked on Microsoft Q&A and IT forums. None of these carry an official Microsoft acknowledgment yet, so treat them as unconfirmed until Microsoft’s support page is updated.
What Microsoft says to do
For the USB audio issue, Microsoft’s support page offers one workaround. “Some customers have reported that they’re able to restore audio in these cases by switching to 2-channel mode,” it notes. A permanent fix is in progress. The Hyper-V/Plan9 issue has no workaround listed yet. For Remote Desktop Services problems, Microsoft is directing affected organizations to contact Microsoft Support for Business. A proper fix is planned for a future update.
Should you still install it?
Yes. The two zero-day vulnerabilities this update fixes were already being exploited. That is a bigger risk to most people than these known issues. Delaying a security patch to dodge a USB audio glitch is the wrong trade for most home users. Microsoft still recommends not delaying quality updates by more than three days. If you rely heavily on Hyper-V Linux VMs, RDS in an office environment, or a domain-joined PC, read Microsoft’s known-issues page before updating. Hold off if your workflow depends on any of the affected features.
The pattern here is getting old
This is at least the second time in 2026 that a monthly Windows 11 update has needed its own known-issues writeup. And it comes right after Microsoft told users not to wait. Consumers end up choosing between an unpatched security hole and a coin-flip on their audio, VPN, or virtual machine still working the next morning. That is not really a choice at all. It is worth Microsoft tightening its testing pipeline before telling users to rush an install.
What to watch
Check Windows Update > Update history for KB5124008’s status on your PC. Keep an eye on Microsoft’s own support page for this KB, which is being updated as fixes roll out. If you hit the USB audio bug, try switching the device to 2-channel mode in Sound settings. Use this as a stopgap until Microsoft ships a proper fix.
Source: Microsoft Support, Windows Latest
