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OS X 10.9 features a new security feature called Gatekeeper which prevents the installation of malware by controlling the types of software that can be installed. According to Apple:
The safest place to download apps for your Mac is the Mac App Store. Apple reviews each app before it's accepted by the store, and if there's ever a problem with an app, Apple can quickly remove it from the store. When you download software from any other place on the Internet, Gatekeeper makes that safer, too. Developers can get a unique Developer ID from Apple and use it to digitally sign their apps. The Developer ID allows Gatekeeper to block apps created by malware developers and to verify that apps haven't been tampered with. If an app was developed by an unknown developer — one with no Developer ID — Gatekeeper can keep your Mac safe by blocking the app from being installed.
By default in 10.9, Gatekeeper is enabled and set to allow only "Mac App Store and identified developers," which prevents some software from installing. When an app installation is blocked, a screen similar to the following is displayed.
To open an application that is blocked from opening by Gatekeeper, please use one of the following workarounds:
For High Sierra, see OSX Daily's: How to Allow Apps from Anywhere in macOS Sierra Gatekeeper
- Navigate to the application you would like to open and right-click or control-click on its icon.
- Select Open from the contextual menu.
Result: You will be prompted with a screen asking if you're sure you want to open the application.
- Select Open.
If you've already tried to run the installer, and received the error above, you can launch the installer by going to
Apple menu > System Preferences > Security & Privacy and click Open Anyway.
- Navigate to System Preferences from the Apple Menu or from the Dock.
- Click on the Security & Privacy preference pane, under the Personal header.
- In the Security & Privacy pane, click on the Lock icon on the bottom-left.
- Enter your Mac's username and password and click Unlock.
- Click the radio button next to "Anywhere".
- When prompted, click Allow From Anywhere.
Result: The radio button next to "Anywhere" will be selected.
- You can now install any compatible application on your Mac without the installation being blocked. Once your application installation is complete, please use the following directions to re-enable Gatekeeper.
Follow the above instructions until step 5, and click the radio button next to "Mac App Store and identified developers".