The Intego blog is reportingthat Snow Leopard appears to contain some sort of built-in antivirus protection. Check out the screenshot below which popped up when a user tried to open up a .dmg file infected with a DNS changer trojan horse.
ZDNet speculates on the matter:
It is not yet clear how Apple is handling the package scans for signs of malicious software.
I have confirmed that Apple is notusing the open-source ClamAV engine to handle these scans so it’s likely the company has entered into an agreement with a commercial anti-virus company.
On that note, some are suggesting that Apple “licensed Symantec’s virus scanning engine” or that it may have even developed its own.
Related: Apple hires noted security expert