nextcloud-mcp-server: Unauthenticated `POST /webhooks/nextcloud` allows arbitrary vector data deletion when `WEBHOOK_SECRET` is unset ( default )

Summary

The POST /webhooks/nextcloud endpoint has no authentication by default: WEBHOOK_SECRET defaults to None and is never required by startup validation. When unset, the receiver accepts any unauthenticated POST. The user_id is taken directly from the attacker-supplied payload and passed to Qdrant, allowing an unauthenticated attacker to delete or corrupt vector embeddings for any user.

Details

Vulnerable file: nextcloud_mcp_server/vector/webhook_receiver.py, function handle_nextcloud_webhook(), lines 55-67

Root cause 1: Auth check is guarded by if secret: - skipped entirely when WEBHOOK_SECRET is unset.

Root cause 2: webhook_secret: str | None = None in config - no startup validator enforces it, even when vector sync is enabled.

Trusted field: payload["user"]["uid"] in webhook_parser.py is used as-is for all Qdrant operations - no cross-check against an authenticated session.

webhook_receiver.py, lines 55-67:

secret = get_settings().webhook_secret  # None by default
if secret:                           # skipped entirely when unset
    ... validate Bearer header ...
else:
    _warn_missing_secret_once()     # just logs, still processes

webhook_parser.py, line 57:

user_id = payload["user"]["uid"]     # attacker-controlled

PoC

No credentials required. Works on any deployment where WEBHOOK_SECRET is not explicitly set (the default).

POST /webhooks/nextcloud
Content-Type: application/json

{
  "event": {
    "class": "OCP\\Files\\Events\\Node\\BeforeNodeDeletedEvent",
    "node": { "path": "/victim/files/Notes/any.md", "id": 12345 }
  },
  "user": { "uid": "victim" },
  "time": 0
}

Result: Qdrant deletes all vector embeddings for victim doc 12345 with no authentication. Attacker can loop over doc IDs for mass deletion. All user targets accepted.

Impact

• Anyone on the network with access to port 8000 - no credentials needed.
• Attacker can delete or trigger re-index of any user's vector embeddings in Qdrant by spoofing user.uid in the payload.
• Mass-sending delete events for all doc IDs destroys the entire semantic search index for all users, requiring a full re-scan to recover.

Recommend Fix

1. Enforce WEBHOOK_SECRET at startup ( file config_validators.py )

if vector_sync_enabled and not settings.webhook_secret:
    raise ConfigurationError(
        "WEBHOOK_SECRET must be set when vector sync is enabled"
    )

2. Reject requests when secret is unset ( file webhook_receiver.py )

secret = get_settings().webhook_secret
if not secret:
    return JSONResponse({"status": "unavailable"}, status_code=503)
provided = request.headers.get("authorization", "").encode()
if not hmac.compare_digest(provided, f"Bearer {secret}".encode()):
    return JSONResponse({"status": "unauthorized"}, status_code=401)