Apple has just released its first public Background Security Improvement (BSI) for macOS 26.3.1 Tahoe, labelled as BSI (a)-25D771280a. Once installed, macOS will identify itself as version 26.3.1 (a), with a build number of 25D771280a.

You can install this through Privacy & Security Settings, in the Background Security Improvements section. It doesn’t appear listed in Software Update, although SilentKnight will offer it. Please don’t try to use SilentKnight to install this, though, as it will download successfully but fail to install unless you then use the BSI section in Privacy & Security settings, which will finish the job off.

Apple has now released details of the single vulnerability that this fixes, in WebKit. As a result it updates Safari from 26.3.1 (21623.2.7.11.7) to 26.3.1 (21623.2.7.111.2).

Following installation, your Mac will need to restart for the BSI to be applied.