PYSEC-2026-3866

## 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**: ```python 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**: ```python user_id = payload["user"]["uid"] # attacker-controlled ``` ## PoC **No credentials required**. Works on any deployment where `WEBHOOK_SECRET` is not explicitly set (the default). ```json 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` ) ```python 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` ) ```python 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) ```