CVE-2026-55640: nextcloud-mcp-server: Unauthenticated `POST /webhooks/nextcloud` allows arbitrary vector data deletion when `WEBHOOK_SECRET` is unset ( default )

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.