Last week, it was claimed by security researchers that Shellshock was active in the wild and could potentially issue commands to a computer which would allow the attacker to gather information and or modify system files.
Most OS X users will be unaffected by the vulnerability, as Shellshock only targets users who have configured advanced UNIX services. So if you use terminal, or you look to spawn a shell like Perl or even use startup scripts, you will be affected. If you do use advanced UNIX services as described here, then you should update your OS X immediately.
You might use the App Store to update your OS X system, but in the event this bash update isn’t showing, you can download the appropriate OS X bash update 1.0 from the following links:
OS X bash Update 1.0 – OS X Mavericks http://support.apple.com/kb/DL1769
OS X bash Update 1.0 – OS X Mountain Lion http://support.apple.com/kb/DL1768
OS X bash Update 1.0 – OS X Lion http://support.apple.com/kb/DL1767