Widespread MacOS installer flaw: “This copy of the Install macOS Mojave application is damaged, and can’t be used to install macOS”

Need to fix a Mac by reinstalling macOS?

If you downloaded its installer before October 2019, you might not be able to.

People who rely on their Macs for their livelihood are well advised to maintain a bag of tricks to help out when things go wrong. A spare drive with vital installers and utilities to fix problems when the Mac they need stops working. One important installer for such a drive is that for macOS.

For those who have been archiving macOS installer applications, there’s a problem. If you downloaded one before October 14th 2019, it probably won’t work any more. Apple macOS installers available before October 2019 were signed with an old certificate, so Macs will not run if their clock is set to after October 24th 2019. For a technical explanation, go to the Der Flounder blog from Rich Trouton, Mac system administrator.

The error message that will appear will state: “This copy of the Install macOS Mojave application is damaged, and can’t be used to install macOS”

[Image from a tweet by Eric Holtam]

Similar messages will come up if you try to run older High Sierra and Sierra installers (even if they install 10.13.6 and 10.12.6 respectively).

Apple have recently updated their OS installers, so the solution is to follow these links and download the macOS installers you need: Apple support documents on updating to macOS Sierra (10.12.6), macOS High Sierra (10.13.6), macOS Mojave (10.14.6).

Once you have the installer(s) you need, don’t forget to update your ‘emergencies drive’!