The CentOS Atomic SIG has released an updated version of CentOS Atomic Host (7.1712), a lean operating system designed to run Linux containers, built from standard CentOS 7 RPMs, and tracking the component versions included in Red Hat Enterprise Linux Atomic Host.
This release includes updated kernel, linux-firmware and microcode_ctl packages to address recent security advisories, alongside other minor updates that shipped during the month of December.
CentOS
I believe the Kernel and Firmware updates are to fix the meltdown bug.