A Windows security switch has been stuck in the off position since Tuesday

Open Windows Security, go to App and browser control, and look at Smart App Control. If it says off and the switch refuses to move, you are not doing it wrong. Microsoft confirmed on 16 October that October's security update broke it.

The update is KB5127648, released on 13 October, which brings Windows 11 to builds 26100.9711, 26200.9711 and 26300.9711 depending on which version you run. Microsoft's own wording in the release notes: "After installing this update, some devices might be unable to turn on Smart App Control in the Windows Security app if it has been turned off." Version 24H2 and everything after it is listed. There is no fix yet and no date for one.

Smart App Control is the feature that checks a program against Microsoft's reputation data before letting it run and blocks anything it has never seen, which is a reasonable line of defence against the downloaded installer that turns out to be an infostealer. The good news, such as it is, hides in one sentence from Microsoft: devices where it is already turned on are not affected and stay protected. So this only bites people who had switched it off at some point and were trying to go back.

Microsoft has published two registry commands and a pair of group policies as a stopgap, both aimed at managed office machines rather than home PCs. For everyone else the honest answer is to wait, and meanwhile not rely on a switch that is currently stuck. Tendvane's health score reports what your Windows protections are actually doing right now, which is more useful than what the settings screen claims.