So we have a legacy server with Debian 6.0 !, the solution is to add LTS repository for Debian. I follow these instructions on turnkeylinux website.

First enable LTS updates on your Debian 6.0:

~# apt-get install -y debian-keyring debian-archive-keyring

~# cat>>/etc/apt/sources.list.d/security.sources.list<<'EOF'
deb squeeze-lts main contrib non-free
deb-src squeeze-lts main contrib non-free

Update repositories and upgrade packages:

~# apt-get update
~# apt-get upgrade

Find which packages are not supported:

~# apt-get install -y debian-security-support
~# check-support-status

Now setup the unattended-upgrades like i described in [my blog], for more info go to debian wiki.