Tente isto:
Unattended-Upgrade::Origins-Pattern {
"o=Debian,a=testing";
"o=Debian,a=stable";
"o=Debian,a=stable-updates";
"o=Debian,a=proposed-updates";
"origin=Debian,archive=stable,label=Debian-Security";
};
O arquivo /etc/apt/apt.conf.d/20auto-upgrades deve conter:
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";