Today a vulnerability was discovered in Bash, the shell used on most Linux distributions. We strongly recommend that you patch your servers as soon as possible against this.
Debian / Ubuntu
$ sudo apt-get update && sudo apt-get install -y bash
CentOS / RedHat Enterprise Linux / Fedora
$ yum update bash
Make sure to compare the updated package with the list of patched versions here.
Update: There has been a lot of development around this in the last 48 hours. The site Shellshock has a lot more information on the issue and how you can protect yourself.