macOS Sequoia 15.7.9 ships two different mBoot firmware versions (August 28, 2026)
Chapters
What a reader found, and what the author confirmed
This surfaced in the comments on the day's Golden Gate piece rather than in an article. At 10:17 AM on August 28, 2026, a commenter named Johnson reported a MacBook Air M3 running macOS Sequoia 15.7.9 that was carrying mBoot firmware 18000.161.10, where the expected version for that build was .9. The thread is on The Eclectic Light Company and is dated August 28, 2026.
The author replied between 11:12 and 11:26 AM, verbatim: 'Apple has changed the firmware supplied in those updates. If you installed the update before 26.6.2, then .9 was installed. If after 26.6.2, then you should expect .10.' So the variable is not the Mac and not the build. It is the date you installed.
TL;DR
Macs reporting the same macOS version and build are carrying different firmware: macOS Sequoia 15.7.9 installed before macOS Tahoe 26.6.2 shipped has mBoot 18000.161.9, and the same build installed after it has 18000.161.10, according to The Eclectic Light Company on August 28, 2026. The same split applies to macOS Sonoma 14.8.9. Apple issued no announcement, no revised build number and no explanation, so firmware checks that compare what a Mac has against what its build should have now flag mismatches that are not faults.
- The two firmware versions are mBoot 18000.161.9 and 18000.161.10, on macOS Sequoia 15.7.9 and macOS Sonoma 14.8.9.
- Which one you get depends on whether you installed the update before or after macOS Tahoe 26.6.2, which shipped on 2026-08-17 carrying 18000.161.10.
- A reader reported a MacBook Air M3 on Sequoia 15.7.9 with 18000.161.10 rather than the expected .9, and the author confirmed the pattern within the hour.
- The author called the firmware situation 'even more of a mess than I had realised', and on this day it was undetermined whether this is a defect or deliberate.
- There is no workaround: a Mac left on .9 cannot be moved to .10 except by the next macOS update.
Where 18000.161.10 came from
macOS Tahoe 26.6.2, build 25G83, shipped on 2026-08-17 and legitimately carried mBoot 18000.161.10. That is the origin of the newer firmware. From that date the security updates for the older systems, macOS Sequoia 15.7.9 and macOS Sonoma 14.8.9, appear to have started supplying .10 as well, without either release getting a new version or build number to mark the change.
For context on the releases involved, the final macOS Sonoma version looks to have been 14.8.9, released 2026-08-06, so for a Sonoma Mac that is likely the last security update on that major version. Either way, two Macs can have taken the same update weeks apart and ended up on different firmware.
| macOS release | Installed before macOS Tahoe 26.6.2 | Installed after macOS Tahoe 26.6.2 |
|---|---|---|
| macOS Sequoia 15.7.9 | mBoot 18000.161.9 | mBoot 18000.161.10 |
| macOS Sonoma 14.8.9 | mBoot 18000.161.9 | mBoot 18000.161.10 |
| macOS Tahoe 26.6.2 (25G83) | Not applicable, released 2026-08-17 | mBoot 18000.161.10 |
Why this breaks firmware checking
Firmware verification works by comparing the firmware a Mac reports against the firmware its macOS build is expected to have. That comparison is now unsound for these releases. A Mac on Sequoia 15.7.9 with 18000.161.10 will be flagged as a mismatch by tooling that expects .9, and the mismatch is not a fault.
The practical loss is a baseline. An administrator cannot derive an expected firmware version from a macOS build number any longer, and a user cannot predict which firmware an update will install. That is a change in how macOS updates behave, and it arrived without documentation.
What Apple has said about it
Nothing. As of August 28, 2026 there was no Apple announcement, no revised build number and no explanation, and whether this is a defect or a deliberate change was undetermined on the day. The author's own assessment of the firmware picture was that it is 'even more of a mess than I had realised'.
It is worth saying plainly that this is not a patchable bug in the usual sense. It is a change in what Apple puts inside an update package, so there is nothing for a point release to fix and no corrective release was announced.
What is coming, and what it does not solve
The next round of releases is close. macOS 27 Golden Gate was expected between 15 and 22 September 2026, with 'greatest likelihood on the same day as the arrival of those new Macs', and macOS Tahoe 26.7 and macOS Sequoia 15.8 were expected alongside it. Golden Gate's own firmware was expected to be approximately 20457.1.29.
Those dates were already visible in the release trains. macOS Sequoia 15.8 had reached release candidate 2, build 24H20, according to The Mac Observer on August 24, 2026. So a Mac sitting on 18000.161.9 will most likely be moved by that update, and until then there is no published way to change its firmware.
What you can do today
1. Record the date you installed the update next to the build number. On these releases the install date, not the build, decides which mBoot version you have.
2. If firmware tooling flags a mismatch on macOS Sequoia 15.7.9 or macOS Sonoma 14.8.9, compare it against both 18000.161.9 and 18000.161.10 before treating it as a fault.
3. Do not try to force a firmware change. There is no published route from .9 to .10 other than installing the next macOS update.
4. If you keep an expected-firmware baseline for a fleet, split it by install date for these two releases rather than by build number, because one expected value per build is now wrong.
5. Expect the mid-September update window to move these machines again, and re-record the baseline after it lands rather than before.
Frequently asked questions
Is a Mac on the older mBoot firmware less secure?
Nothing published on August 28, 2026 says so. Apple did not describe the change, so there is no statement about what differs between 18000.161.9 and 18000.161.10, and no advisory to read. The reported problem is the unpredictability, not a stated vulnerability.
Which Macs are involved?
Apple silicon Macs taking the macOS Sequoia 15.7.9 or macOS Sonoma 14.8.9 security updates. The reader case was a MacBook Air M3. No Mac model is singled out as affected or exempt.