Yesterday Apple released patches for their three most recent operating systems (Mavericks, Mountain Lion and Lion) to secure your computers against potential Bash/ShellShock security exploits. It’s highly recommended that you download and install the appropriate patch for your Macs ASAP.
If you’re interested in learning what the ShellShock/Bash exploit is all about, here are a few detailed articles on it: Ars Technica, Symantec.
OS X Bash Update – Lion (10.7)
OS X Bash Update – Mountain Lion (10.8)