Gatekeeper is a security feature that Apple introduced with OS X 10.7.5 that keeps you from running potentially harmful software on your Mac. You've probably encountered it when you first open an app that's been downloaded from the Internet and you're asked to confirm that you want to open it. You may also be familiar with the practice of having to right click and select "Open" for some apps that aren't signed by the developer.
However, in rare cases you may run into an issue where OS X tells you that an app you've downloaded is damaged and can't be open. To get around this issue, there's a bit more that needs to be done.
Gatekeeper's Scary "trash this app" warning
Typically if an app isn't signed when you go to open it, you can simply right click its icon and click "Open" as described in this tip from Melissa Holt. However, if you get a warning that says something like "App name is damaged and can't be opened. You should move it to the Trash," you should be cautious about going any further. Apple's warning is worth heeding, especially if you don't know where the app came from. However, if you're absolutely certain that you want to take the risk, there are a few more steps involved in fixing the issue:
- Open System Preferences from the Apple menu.
- Select Security & Privacy.
- Select the General tab.
- If it's not unlocked, click the lock in the lower left corner and enter an administrator's password to unlock the preference pane.
- Under "Allow apps downloaded from," select "Anywhere."
- Open the app
Security & Privacy Preference Pane
Once you confirm that the app is up and running, return to the preference pane and switch the "Allow apps downloaded from" setting back to either "Mac App Store and identified developers" or "Mac App Store." This way your Mac remains secure and subsequent openings of the offending app will work just fine without you having to toggle the settings back and forth. Enjoy