Security researchers from F-Secure have spotted a Mac OS X malware that disables Snow Leopard's XProtect.plist antivirus signatures updates.
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Recent analysis has revealed to us that Trojan-Downloader:OSX/Flashback.C disables the automatic updater component of XProtect, Apple's built-in OS X anti-malware application.
Apple added a built-in antivirus scanner in Snow Leopard in August, 2009. Back then, the feature only scanned for two trojans.