How can we upgrade bash on our instances? This is a nasty vulnerability and should be addressed ASAP.
http://www.zdnet.com/shellshock-how-to-protect-your-unix-linux-and-mac-servers-7000034072/ https://securityblog.redhat.com/2014/09/26/frequently-asked-questions-about-the-shellshock-bash-flaws/
Test your instance like this -- if you see "vulnerable" echoed back, you're vulnerable.
env VAR='() { :;}; echo vulnerable' bash -c "echo Bash Test"