The Apple Gatekeeper is designed to protect OS X users by performing a number of checks before allowing an App to run. Apple tried to mitigate the attack method (CVE-2015-7024) with the release of a new OS version, the OS X El Capitan 10.11.1.1. An attacker can use a malware that remain silent during the checks, then it activates the malicious code. The attacker delivers the malicious.dmg file to users by injecting it into insecure download connections or by spreading it using third-party app stores.”]
Source: http://securityaffairs.co/wordpress/43632/hacking/bypass-apple-gatekeeper.html