I have created a scripts that handle these tasks for you, available `here`_.
First thing we need to do is create an sources list specifically for security.
sudo grep "-security" /etc/apt/sources.list | sudo grep -v "#" > /etc/apt/security.sources.list
Now that this is done we can simply continue to use the command below to trigger security-only upgrades
sudo apt-get upgrade -o Dir::Etc::SourceList=/etc/apt/security.sources.list
This will work until you upgrade your distro (e.g. 10.04 -> 12.04), at which point you will need to re-run the first command to regenerate the security.sources.list file.