Back in February, Apple's Worldwide Developer Certificate, designed to verify third-party apps and services,
expired, requiring Apple to issue a new certificate for developers to use.
As pointed out by
TidBITS (via
Ars Technica) a side effect of the replacement of the certificate causes older OS X installers to fail to launch. OS X installation files downloaded from the Mac App Store before February 14, 2016 and stored on a computer or USB drive are no longer functional. This includes installation files for OS X El Capitan and older versions of OS X like Mavericks and Mountain Lion.
Users who keep OS X installers on hand or have created USB install disks in case of emergencies should replace their files with new versions by re-downloading them through the Mac App Store using purchased history.
TidBITS points out that some older versions of OS X, like Lion, will not be downloadable on newer machines. Users who can't replace an expired certificate can still use older installers by changing the dates on their machines.Apple's certificate renewal process resulted in some problems for developers and Mac users back in November, as it caused multiple Mac App Store apps to display a "damaged" error and fail to open. Apple
quickly addressed the issue and has helped developers transition to the new certificate.
Article Link:
OS X Installers Downloaded Prior to February 14 No Longer Work After Certificate Update