Apple has introduced a number of features designed to protect users from malware in OS X, but these tools occasionally go too far when trying to save people from themselves.
OS X's Gatekeeper feature -- introduced with OS X Mountain Lion -- places restrictions on which apps can be run on a Mac based on the avenue through which the apps were acquired. There are three tiers: apps which are distributed by registered developers through the Mac App Store, apps which are distributed by registered developers outside of the Mac App Store, and apps which are not made by registered developers.
Gatekeeper distinguishes between the latter two based, broadly, on whether the app has been signed with a legitimate Apple-issued signing key.
By default, Gatekeeper is configured to allow apps from the Mac App Store and from registered developers. Users can make this more or less strict:
- Open System Preferences
- Open the "Security & Privacy" pane
- Select the "General" tab
- Click the lock icon in the lower-left corner and enter an administrative username and password
- Select one of the three available levels under "Allow apps downloaded from:" and close the preference pane
Unless you choose to allow apps downloaded from anywhere, OS X will warn you against opening apps that aren't signed: you'll see a dialog box that says "
If you're sure the app is safe, you don't need to alter your security preferences to open it -- there's a faster workaround.
Right click (or command-click) on the app and select "Open" from the context menu. This will present a slightly different dialog box: this time, you'll be presented with an "Open" button that will let you force OS X to run the app.
Remember: only do this if you're sure the app is from a reputable developer and has not been tampered with.