The latest annoyance in Xcode had me stumped for a bit but @smappsoft set me straight. When trying to build and app under iOS 8, the build succeeds but the app doesn’t launch. Saying “process launch failed: Security” and not much else.
Turns out that the build was successful but your shiny new iOS 8 device doesn’t “trust” your profile.
Security: 1, Developer: 0
So to solve this new cryptic behavior, launch the app on your device (which you just installed) and you will get a trust prompt asking you to approve an “Untrusted App Developer”. This is Gatekeeper asking you to confirm your trust of the developer (yourself).
Once you’ve completed this trust you can happily install apps from Xcode.