The security hole this time is with how Debian and Ubuntu, and almost certainly other Linux distributions, implement Linux Unified Key Setup-on-disk-format (LUKS). LUKS is the standard mechanism for implementing Linux hard disk encryption. LUKS is often put into action with Cryptsetup. It's in Cryptsetup default configuration file that the problem lies and it's a nasty one.
As described in the security report, CVE-2016-4484, the hole allows attackers "to obtain a root initramfs [initial RAM file system] shell on affected systems. The vulnerability is very reliable because it doesn't depend on specific systems or configurations. Attackers can copy, modify, or destroy the hard disc as well as set up the network to exflitrate data. This vulnerability is specially serious in environments like libraries, ATMs, airport machines, labs, etc, where the whole boot process is protected (password in BIOS and GRUB) and we only have a keyboard or/and a mouse."