Unattended upgrades on Debian allow to sync/update any repository and install the upgraded packages automatically. This makes sense for packages in the security repository.

The following commands enable unattended security upgrades on Debian 7 and 8. Note that it is not enough to just install the "unattended-upgrades" package.

apt-get install unattended-upgrades apt-listchanges

dpkg-reconfigure -plow unattended-upgrades

By default it runs daily, sends a mail to root. Follow the Debian Wiki for further teaking: https://wiki.debian.org/UnattendedUpgrades