A critically severe vulnerability affecting Linux GNU C Library (glibc) was announced and is known as GHOST (CVE-2015-0235).
We highly recommend that you update glibc on your server and schedule a full system reboot to be completely safe from any exploit related to this
Instructions to update,
For Ubuntu/Debian, you can run:
apt-get update && sudo apt-get install –only-upgrade libc6 -y
For Centos/RHEL, you can run:
yum update glibc
You can read more about this announcement here, http://www.openwall.com/lists/oss-security/2015/01/27/9