Learn more about what to do with your MacOS when you want to open a Disk Image or an installer package from a non-App Store source.

What to do

MacOS uses a security system called “Gatekeeper” to protect users from potentially untrustworthy applications – especially those distributed outside of its own Apple App Store (see https://support.apple.com/en-gb/ht202491 for more details).

Gatekeeper is triggered when a macOS Disk Image or Installer is downloaded from somewhere other than the Apple App Store (for example, downloaded from the Internet or filestore, or has been received by e-mail). Double-clicking on the file will produce a “can’t be opened” dialog box:

Step 1

If you are certain that you can trust this Disk Image or Installer Package, the simplest way to allow the file to be opened/run is to Ctrl+Single Click the file and choose “Open” from the dialog box that pops up - or use the Right Click if your secondary mouse button has been set up to do this:

Step 2

This will present you with an “Are you sure?” dialog, instead. Again, if you’re certain that the file can be trusted, click “Open”:

Step 3

Once opened, many applications will have been “signed”. This is a good double-check of who they were written by and that the contents have not been interfered with since they were created. Please remember this check does not guarantee what the contents of the package will actually do!

Useful downloads

You will find a downloadable PDF of this guide at the top of this page.