Mac OS X Snow Leopard and malware detection
"Malware" is an abbreviated term for malicious software. Malware includes viruses, worms, trojan horses, and other types of software that can damage the software on your system or violate your privacy. Malware can be installed on your computer when you download content or applications from the Internet, via email, text messaging, or websites. Mac OS X v10.6 Snow Leopard checks for known malware and alerts you so that you do not accidentally install it on your system.
Files downloaded via applications such as Safari, iChat, and Mail are checked for safety at the time that they are opened. If a file is identified as containing known malware, the system will display a dialog that alerts you to move it to the Trash. You should empty the Trash to finalize the removal of the file.
Apple maintains a list of known malicious software that is used during the safe download check to determine if a file contains malicious software. The list is stored locally, and with Security Update 2011-003 is updated daily by a background process.
If you do not wish to receive these updates, you can disable daily update by unchecking "Automatically update safe downloads list" in the Security pane, in System Preferences. This option appears in Security preferences after Security Update 2011-003 is installed.
Security Update 2011-003 provides additional protection by checking for the MacDefender malware and its known variants. If MacDefender malware is found, the system will quit this malware, delete any persistent files, and correct any modifications made to configuration or login files. After MacDefender is identified and removed, the message below will be displayed the next time an administrator account logs in.