While Apple has released a security update to address the “shellshock” vulnerability in the bash shell, they have not made it available through Software Update!
See the support page at: http://support.apple.com/kb/HT1222 for links to the downloads and installation instructions.
Update: Apple has rolled the bash shell update into Security Update 2014-005. See the details at: https://support.apple.com/kb/HT6531
Presumably, the fix is also included in OS X 10.10 “Yosemite” (https://support.apple.com/kb/HT6535) which was released yesterday.