There is a very critical security problem detected with linux server which allow hackers to gain access to your Mailserver and SSH without ever knowing it’s password.
It is a buffer overflow caused by Glibc known as GHOST, it is vulnerability # CVE-2015-0235 which announced on 27th January. More details about this issue is here.
You can patch your C Library (Glibc in CentOS or libc in Debian) easily know. they have published fix and all you need to do is to update your server and then reboot it.
In CentOS (5,6,7) you can run following commands to fix this issue:
yum update glibc
and then you need to reboot your server:
shutdown -r now
And you are on safe side now!
In Debian or Ubuntu, you can use:
apt-get update apt-get upgrade
to update your OS.
again, type reboot command to reboot your server:
and now your servers are safe 🙂