cloud-init through 25.1.2 includes the systemd socket unit cloud-init-hotplugd.socket with default SocketMode that grants 0666 permissions, making it world-writable. This is used for the "/run/cloud-init/hook-hotplug-cmd" FIFO. An unprivileged user could trigger hotplug-hook commands.
https://github.com/canonical/cloud-init/releases/tag/25.1.3
https://github.com/canonical/cloud-init/pull/6265/commits/6e10240a7f0a2d6110b398640b3fd46cfa9a7cf3