CVE-2026-78408 — Util-linux: nsenter --join-cgroup leaks root cgroup migration authority

The util-linux nsenter --join-cgroup option can leave a target cgroup.procs file descriptor opened as root across later namespace and credential changes and across execve(). After a privileged operator uses this option against an attacker-controlled target, an unprivileged user may be able to migrate and terminate unrelated root processes.