Some apps that are downloaded and installed from the Internet could adversely affect your Mac. Gatekeeper helps protect your Mac from such apps. It is designed to protect Mac users from running malicious software by checking apps before they run for the very first time. Gatekeeper is Apple’s answer to Trojan apps that trick users into installing software they don’t actually want.

By default, with MAC OS X 10.8 Mountain Lion, If you tried to install an application that is not installed from the App Store, then you will receive a pop up  message stating that the application can't be opened because it is from an unidentified developer.

For example, I'm trying to install FileZilla, which is an open source FTP Client ! The application is neither downloaded from the App Store, nor from a registered Developer with Apple, hence, Gatekeeper blocked me from installing suck Application.



In such case, you have two options: Either to bypass Gatekeeper temporarily per application, or permanently for all applications.

It’s easy to manually override Gatekeeper on a case-by-case basis without changing the default setting. Just right-click (or control+click) the app and select Open from the menu. Gatekeeper will still display a warning, but it will also give you the option to install the app anyway.  However, if you want to change Gatekeeper settings or even disable it, then follow the below steps :

1. Open System Preferences. Do this by clicking on the Apple logo   in the top left side of the screen and then click on System Preferences...
   
   
   
   

2. In System Preferences and under the Personal section, click on Security & Privacy
   
   
   

3. Under the General tab, you can find the gatekeeper option:
   
   Allow applications downloaded from:

• Mac App Store : Only apps that are downloaded from the Mac App Store can be opened

• Mac App Store and identified developers : (default in OS X Mountain Lion)  Only apps that are downloaded from the Mac App Store and  applications from the Web/Internet, signed by certified Apple developers
  
  Note : Developers are not forced to register with Apple to be identifiable by Gatekeeper. Apple only recently open the registration and it is still unknown if app developers will choose to register with Apple if they are not selling their apps via Mac App Store. Thus it does not mean that the app is not trustworthy to be run if it is not on the Mac App Store or not from an Apple registered developers

• Anywhere : Allow applications to run regardless of their source on the Internet (default in OS X Lion v10.7.5); Gatekeeper is effectively turned off.

So, with the default settings, if you want to download and install application not from the Mac Store and not from unidentified developers, then the application installer will be blocked.

4. To make Changes, click on the lock at the bottom of the preferences page and type the username and password of a user account with administrative rights.
   
   
   

5. Once you authenticate, the options will not be grayed anymore and you can start making changes. For example, to install applications from the internet where they are not from the App Store and not from an identified developer, then select Anywhere
   
   
   
   You will be presented with a warning message that enabling Anywhere, will make your Mac less secure. To confirm your selection, click Allow From Anywhere
   
   
   

6. Now, go back and run the application that you downloaded not from the App Store and not from identified developer.
   
   MAC OS X this time will not block the installer, but will give you a message stating that this application was downloaded from the Internet, and if you are sure to open it. Click Open
   
   

 

Summary

Gatekeeper is a new feature, it is designed to protect users from dangerous applications. Specifically, blocking applications that are not downloaded from the App Store or from unidentified Developers.

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