According to Apple’s release notes, High Sierra 10.13.6 updates brings one new feature (AirPlay 2 multi-room audio support for iTunes), two bug fixes (in AVCHD support and Mail), and nine security fixes, as well as new versions of Safari (11.1.2) with security fixes, and a separate update to iTunes (12.8).
Apple does not mention that these also bring EFI firmware updates to many of the more recent Mac models, including:
- iMac16,1, but not iMac16,2
- iMac18,1, iMac18,3
- MacBookPro13,2, MacBookPro13,3
- MacBookPro14,1, MacBookPro14,2, MacBookPro14,3
Nor is there any explanation as to what those EFI firmware updates actually change. However, some at least match the firmware versions reported by those using macOS Mojave betas, and may be in preparation for the Mojave release in the autumn/fall. These firmware updates appear to be installed by either the 10.13.6 update, or Security Update 2018-004, so should apply whether you are running Sierra or High Sierra (and probably El Capitan too).
Those new firmware versions are listed here.
High Sierra 10.13.6
This is likely to be the final release of High Sierra before Mojave, although Apple has occasionally provided later updates in the past. So as far as 10.13 goes, this is probably going to be as good as it gets apart from future security updates.
The 10.13.6 update is quite large, ranging in size between about 1.6 and 1.9 GB, depending on how it is obtained. According to Suspicious Package, the delta update installs a total of over 36,000 items, which is just over 4 GB on disk. This is, once again, a substantial update which would appear to go considerably beyond the few changes revealed by Apple.
Apps which are replaced include:
- App Store
- Safari (11.1.2)
- AirPort Utility
- Disk Utility
- Keychain Access.
This update replaces relatively few command tools in /bin or /sbin, but many in /usr/bin and /usr/sbin, and in /usr/libexec. Notable among those is
eficheck, and there is a new set of Allow Lists for that tool.
APFS is replaced with version 748.51.0 again, but there has been no change in its version number since that in 10.13.4, despite fixing a security bug in 10.13.6, and replacing all nine of its command tools and its extension. This confirms that High Sierra is very unlikely to ever support Fusion Drives – for those, you will need Mojave, provided that Apple doesn’t pull their support at the last minute, as it did in High Sierra.
XProtect remains at version 2099, which has been unchanged since its last update on 13 March 2018 – a surprisingly long time ago given the updates made to other security systems in macOS since.
There are plenty of changed apps and tools in /System/Library/CoreServices, though, with replacement of almost all Apple-supplied extensions (KEXTs). The latter has been a feature of several of the updates to High Sierra, and is unlikely to indicate any significant functional change.
Security Update 2018-004
The bulk of the payload in this update consists of EFI firmware updates and a large set of replacement extensions. Other than Safari and iTunes, there don’t appear to be any significant app replacements or updates.
Interestingly, APFS support is updated, but remains on its existing beta track which is probably still incompatible with the first full release in High Sierra.
This is likely to be the last security update provided for El Capitan, if Apple adheres to its existing policy.