NC 3D Print is designed so browsers cannot push arbitrary G-code at a printer, and so the slicing engine (which parses untrusted meshes) stays sandboxed.

• App use is gated to Nextcloud admins or members of the configured allowed_groups (empty ⇒ admins only).
• Disruptive Moonraker updates (UpdateController) are admin-only, idle-only, and target-allowlisted.

• Primary: Admin Settings configure slicer + Moonraker (+ optional multi-printer JSON). Those values are the durable source of truth.
• Session / discovery: allowed users may register a Moonraker URL for the browser session after LAN discovery (SessionPrinterController). This is intentional for “Found on network → use now.” Hosts are filtered by UrlSafety (blocks loopback, link-local, cloud metadata; allows RFC1918 / normal DNS). camera_url is also UrlSafety-checked at register time.
• Path/query strings on proxies cannot retarget hosts (http:// in query is rejected).

• GET/HEAD: monitoring prefixes only (server/info, server/files/, printer/objects/, printer/print/ status, temperature store, history, job queue list, timelapse, device_power list, spoolman, webcams, update status, announcements).
• POST: only server/job_queue/ and machine/device_power/ (UI queue + power toggles).
• PUT / PATCH / DELETE: denied on the proxy.
• Raw printer/gcode/script is never on the allowlist. Guarded writes (temps, tuning, e-stop, filament, bed mesh, …) go through PrinterController with clamps and idle/motion guards.
• G-code console send is off by default and can only be enabled via occ (console_enabled) — not via the Admin settings API.

• Explicit prefix allowlist (api/health, api/slice, api/jobs/, api/mesh/, api/project/, api/flush, …) — not blanket api/*.
• Traversal segments blocked via PathSanitizer; upload bodies capped (50 MB).
• Sidecar is internal-only on nc-print-net (no published host port), cap_drop: ALL, read-only rootfs + tmpfs scratch.

If Admin points slicer_internal_url at a host-bound forge-slicer on :8766, InternalUrlResolver may rewrite to LAN :8082 for Docker reachability. Prefer the owned http://nc-print-slicer:8080 sidecar (make slicer-up).