Mac malware is a serious threat to the security of users’ computers and information. The same protection mechanism can be used on existing malwares that are already detected by Anti-malware products, to make them completely undetectable. Apple implements internally an encryption mechanism to protect some of their own executable like Dock.app or Finder.app. This encryption can be applied to malware as well. If one does, anti-malmalware solutions can no longer detect the malware because of the encryption, but OS X has no problem loading such malware.
Source: https://thehackernews.com/2013/10/apples-own-encryption-mechanism-allows.html