fix(web): refuse cross-site state-changing requests (Origin/Referer check) (#674)

The web interface refuses state-changing requests (POST/PUT/PATCH/DELETE)
whose Origin (or, without one, Referer) is not the host they were sent to,
or is null: 403 CROSS_SITE_REQUEST (web_interface/origin_guard.py). Any
website a LAN user visited could otherwise make their browser POST a plain
form to the Pi. /api/v3/system/action also refuses form-encoded and
text/plain bodies (415) unless sent by HTMX. Clients that send no Origin or
Referer (curl, requests, Home Assistant, the MQTT bridge) are unaffected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/system.py

@@ -383,16 +383,27 @@ def _perform_core_update_locked(stash_local_changes=True):
 def execute_system_action():
     """Execute system actions (start/stop/reboot/etc)"""
     try:
-        # HTMX sends data as form data, not JSON
-        data = request.get_json(silent=True) or {}
-        if not data:
-            # Try to get from form data if JSON fails
+        data = request.get_json(silent=True)
+        if data is None and not request.is_json:
+            # Every caller in the interface sends JSON (the Quick Actions
+            # buttons use HTMX's json-enc). A form-encoded body is what a
+            # cross-site HTML form can send without a CORS preflight, and
+            # this route reboots, powers off and pulls code, so it is only
+            # accepted from HTMX: a cross-site form cannot set HX-Request.
+            # This backs up the app-wide Origin check (origin_guard.py).
+            if not request.headers.get('HX-Request'):
+                return jsonify({
+                    'status': 'error',
+                    'message': ('Send the action as JSON '
+                                '(Content-Type: application/json), '
+                                'e.g. {"action": "restart_display_service"}'),
+                }), 415
             data = {
                 'action': request.form.get('action'),
                 'mode': request.form.get('mode')
             }
 
-        if not data or 'action' not in data:
+        if not isinstance(data, dict) or not data.get('action'):
             return jsonify({'status': 'error', 'message': 'Action required'}), 400
 
         action = data['action']