Gears & Gadgets

Some Big Sur users are unable to update macOS due to an MDM bug

Some Big Sur users are unable to update macOS due to an MDM bug

Earlier this week, various macOS sysadmins began reporting that some Apple devices were failing to upgrade to Big Sur version 11.1. Affected users may briefly see the 11.1 upgrade, but then it disappears from the update UI and from the command line softwareupdate tool despite not having been installed.

Erroneous update request

Developer Victor Vrantchan, of the open source MicroMDM project, says he’s found the problem. Some Macs are erroneously requesting 11.0.1 instead of 11.1 from Apple’s update server—which update is then rejected if the machine in question is already running 11.0.1 or newer, as most such are.

Apple's update servers don't like it when you ask for a system update that's older than your currently installed version.
Enlarge / Apple’s update servers don’t like it when you ask for a system update that’s older than your currently installed version.

While the erroneous requests themselves are clear enough, the breaking condition is less so. According to Vrantchan, making the erroneous request “somehow corrupts the state of the software update process,” resulting in a system that no longer presents the update as an option in System Preferences at all.

Restarting the affected system will make the update visible again—but that may or may not help; if the next attempt to install 11.1 generates the same erroneous 11.0.1 request, the problem behavior will simply repeat itself.

Removing the MDM profile—for you non-Mac folks and non-sysadmins, that’s Apple’s misleadingly named Mobile Device Management service, which allows education and enterprise orgs to manage and “supervise” fleets of macOS and iOS devices—also makes the update visible. That said, mortals should not try this workaround lightly; messing with MDM has potentially severe impacts on system security, and there’s no guarantee that re-enrolling the Mac in its workplace will go smoothly, either.

Can I just download the 11.1 update manually?

One seemingly obvious workaround would be to just directly download and manually apply the update package for Big Sur 11.1. Unfortunately, Apple has deprecated combo update packages—there is no manual package to download or install.

As of macOS Big Sur, the format of the updates has changed—and they can no longer be installed without Internet access. Update packages must come directly from Apple, and they must be managed by the softwareupdate process itself. It isn’t even possible to use a custom URL for softwareupdate anymore.

How—or when—can I fix this?

Until we have more concrete information on the condition causing softwareupdate to erroneously request 11.0.1 instead of 11.1, there’s no guaranteed way to fix the issue. Many users are reporting that after rebooting and attempting to upgrade again—in some cases, after rebooting and trying several times—the upgrade does go through properly.

Other users report that removing the MDM profile fixed the issue entirely—though we’d still consider that a very risky, Hail Mary play.

With no ability to manually download and install system updates as user-installed packages in Big Sur, getting the bug patched might be tricky. The bug might not manifest when the system requests a different version number, so perhaps an update to 11.2 will bypass the buggy requests and therefore be able to fix the bug in 11.2 without intervention. It would also be possible—if extremely hacky—for Apple to sideload the update into an unrelated patch that doesn’t affect the OS version number.

All else failing, a complete reinstall of Big Sur from the App Store should get you to 11.1—although it’s a 12.6GB download, and as with any major OS installation, you should be absolutely certain you’ve got good backups of all your data prior to the attempt.

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Tech – Ars Technica

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