ASA-2018-00092 – systemd: Privilege escalation by following non-terminal symlinks

Before version v240, the systemd-tmpfiles program will follow symlinks present in a non-terminal path component while adjusting permissions and ownership. Often -- and particularly with "Z" type entries -- an attacker can introduce such a symlink and take control of arbitrary files on the system to gain root. The "fs.protected_symlinks" sysctl does not prevent this attack. Version v239 contained a partial fix, but only for the easy-to-exploit recursive "Z" type entries.