Microsoft has confirmed that the October 2026 Windows 11 security update (KB5127648) can stop you turning Smart App Control back on. No permanent fix has been released yet.

• What it is: Smart App Control is a Windows 11 protection that blocks untrusted or unknown apps. After the update, the switch can appear greyed out in the Windows Security app if the feature was previously turned off.
• Who is affected: Windows 11 devices (version 24H2 and later) that installed KB5127648 and had Smart App Control switched off. Devices where it is already on are not affected.
• The risk: Affected computers cannot use this extra layer of protection until the issue is resolved. It is not a data loss problem.
• What to do: Do not uninstall the security update, as it contains important fixes. Microsoft has published a temporary workaround using a registry change for individual PCs, and a Group Policy option for managed fleets.

We recommend keeping the October update installed and contacting us if you would like the workaround applied across your devices while Microsoft works on a fix.

Source: BleepingComputer