Since the introduction of the Signed System Volume in Big Sur, the great majority of macOS has been strongly protected. So strongly that applying the smallest security patch has required the full might of a macOS update. There are times when something more lightweight enables Apple to promulgate urgent patches swiftly and efficiently, and that’s what a Background Security Improvement or BSI does.

This was set up when macOS Monterey introduced cryptexes to contain Safari, its WebKit supporting library, and the large dyld caches for general support in Frameworks. Cryptexes are cryptographically sealed disk images that aren’t mounted like other volumes, but are grafted into arbitrary locations in the file system. In Ventura they were used for Rapid Security Responses (RSR), in many ways indistinguishable from BSIs.

This week’s first BSI for macOS 26.3.1 is a good example: it fixes one serious vulnerability in WebKit. Rather than building that into a full update to 26.3.2, because it only requires changes in the cryptex containing Safari and WebKit, this BSI swaps out the existing App cryptex and replaces it with a patched one. For those who don’t want to install BSIs, those same vulnerabilities should be fixed in the next set of security updates to macOS.

Controls

Look in Software Update settings, and you’ll see no mention of any BSI, and that will claim your Mac is up to date, even though it’s not.

BSIs are controlled in their section listed close to the foot of Privacy & Security settings. If you want your Mac to be offered BSIs when they’re available, you must enable Automatically Install first. Despite those words, BSIs don’t appear to install in the least bit automatically, and you should be offered those available for the installed version of macOS. When you’ve chosen to download and install one and authenticated, you’ll see a progress spinner rather than a bar.

As soon as downloading and preparation are complete, you should be given a few seconds before your Mac restarts to complete the installation. This is all very brief, but once you’ve authenticated to start the process, it will run through to completion automatically.

Once your Mac has restarted, you always retain the option to remove any BSI and return to an unpatched cryptex. To see that, click on the ⓘ Info button on the right.

If you decide you want to remove the BSI, your Mac will need to be restarted.

Problems

If you know a BSI is available but Privacy & Security settings appear unable to find it, something I’ve encountered in Virtual Machines, try running SilentKnight. Although BSIs aren’t controlled in Software Update, they do still use the same softwareupdate system used by SilentKnight. Normally you shouldn’t try to install BSIs using SilentKnight, as installation will fail. However, you can turn this to your advantage when a BSI is being elusive.

Once SilentKnight has downloaded and failed to install the BSI, you should be notified of that failure. Restart your Mac, give it a couple of minutes to settle once you’ve logged back in, and open the BSI section in Privacy & Security settings again. The downloaded BSI should now be available, and shouldn’t even need to be downloaded.

If you think a BSI has caused another problem, such as instability in Safari, use the ⓘ Info button to remove that BSI.

Installing a BSI does weird things to the macOS version and build numbers, and those can break scripts and possibly some apps. While ProcessInfo.processInfo.operatingSystemVersion doesn’t contain a field for the BSI letter, ProcessInfo.processInfo.operatingSystemVersionString does return a full version description including the BSI letter and extended build number. In Terminal, sw_vers -productVersion returns the regular version number without BSI, while sw_vers -productVersionExtra returns the BSI designation alone.

Currently, SilentKnight and Skint ignore BSIs, and won’t inform you if you could have one installed except by listing it as an available installation, nor will they check whether your Mac is up to date with the latest BSI. Experience from RSRs in Ventura shows that trying to track lightweight updates like RSRs or BSIs is only going to annoy those who don’t want to install them, and as they can change in a short period, they are hard to track reliably. SilentKnight does report the full version and build number, and SystHist lists details of all BSIs that Mac has installed.

Limitations

Like the RSRs of Ventura, BSIs can only work for a limited range of patches. If a vulnerability needs a fix outside Safari, WebKit, and the dyld caches, then it will require a full macOS update to fix it. BSIs are only ever likely to be provided for the current version of the latest major version of macOS.

From its first account of RSRs, Apple has claimed that some RSRs and BSIs shouldn’t require a restart to apply their patches. However, every RSR and BSI to date has had to be completed by restarting that Mac, which is mildly disruptive and not as lightweight as we’d like.

If you disable Automatically Install in the BSI section of Privacy & Security settings, then your Mac won’t be informed about or have access to any BSIs.

Under the hood

Despite their control being part of Privacy & Security settings, BSIs are managed like all other macOS and related updates by softwareupdated. What is most remarkable about them is their speed of download, preparation and installation compared with macOS updates. From detection of a new BSI to logging back into the restarted Mac can take little more than five minutes.

Apple’s in-house term for BSIs is the same as it used for RSRs, Splat. You’ll also come across Semi-splat, which should be a transient state in which the Splat Restore Version is different from the Cryptex1 Restore Version. That’s normally rectified after the reboot.

softwareupdated checks specifically for BSIs by scanning the update server catalogue for Splat updates. In this case, for an App cryptex, the download size is given as 214 MB. There’s a brief preflight phase, followed by its download. Although no progress indicator is shown in Privacy & Security settings, softwareupdated does record progress, but using similar figures for a full macOS update. Under those, preparing the update is set at 60% progress.

Applying the update takes around 2.5 seconds, at which stage softwareupdated reports that Semi-splat is active because of unequal restore versions, and rollback objects are checked.

Once the Mac has restarted, property list paths are checked for six different Splat versions, enabling the restore versions to be rectified and Semi-splat is no longer active. A brief purge of update assets is performed, and softwareupdated checks once again for any available updates.

Is a BSI just an RSR in disguise?

Apart from the move of its control from Software Update to Privacy & Security settings, there appear to be few if any differences between them. This is even reflected in version numbering. Installing the first RSR for macOS 13.3.1 brought it to version 13.3.1 (a), with a build number of 22E772610a. This first BSI for macOS 26.3.1 brings it to version 26.3.1 (a), with a build number of 25D771280a.

Most telling, though, are the accounts of RSRs and BSIs given in Apple’s Platform Security Guide, which are almost word-for-word identical apart from their names. It seems most likely that a BSI is a rebranded RSR in a bid to move on from the loss of confidence in RSRs following unfortunate errors nearly three years ago.

Key points

• If you’re running the current version of the latest major version of macOS, BSIs provide lightweight fixes for some vulnerabilities, including those in Safari and WebKit.
• Enable them in Privacy & Security settings, in their section at the foot. If they aren’t enabled there, you won’t be offered them at all.
• Control their installation in that section. Once you’ve agreed to install one and have authenticated, your Mac is likely to restart automatically soon after the BSI has been downloaded.
• Remove and revert a troublesome BSI using the ⓘ Info button there.

Apple’s documentation

Support note about BSIs
List of BSIs by date
Security release notes for BSIs

Although the US English version of Apple’s Platform Security Guide has replaced its section on RSRs with an almost identical account of BSIs, most other localised versions of that guide still contain the old RSR version.

Previously

What is a Rapid Security Response (RSR)?
How an RSR went badly wrong