There are several methods to upgrade only security updates on a debian based system, but this one is my favorite.

First i duplicate the sources.list and remove all entries but the lines with the security repositories

sudo cp /etc/apt/sources.list /etc/apt/security.sources.list

Second i ran these easy to remember command

sudo apt-get upgrade -o Dir::Etc::SourceList=/etc/apt/security.sources.list

Thats it. ;)