On OSX? That would be a world-wide first so please quote a specific example we should be aware of?
Windows and OSX are fundamentally different in this regard so please dont extrapolate from Windows viruses and assume the same method is exposed under OSX. ALL known trojans, malware and adware on the OSX platform (inc the latest "Rootpipe" require admin permissions to install being granted by the user in some way.
Unless you have an example?
The Flashback malware would install itself on a Mac just by visiting a compromised/infected web site. No admin password was required.