The system must allow only applications downloaded from the App Store to run. Gatekeeper settings must be configured correctly to only allow the system to run applications downloaded from the Mac App Store. Administrator users will still have the option to override these settings on a per app basis. Gatekeeper is a security feature that ensures that applications must be digitally signed by an Apple issued certificate in order to run. Digital signatures allow Mac OS X to verify that the application has not been modified by a malicious third party.