fix(security): widget API only accepts messages from the widget's own frame

matrix-widget-api's host transport handled a message from ANY window on the
page as long as it carried the widget's id; its strictOriginCheck only
compares with the host's own origin and is off by default. The call's id is
the fixed 'call-embed', so any other frame (a room widget, a URL-preview
embed) could post fromWidget actions as the call. Reproduced locally: an
opaque-origin frame posting one io.lotus.hotkey keydown for the PTT key
turned a push-to-talk user's mic on ("● Live").

restrictWidgetMessages() swaps each ClientWidgetApi transport's listener
for one that requires ev.source === the widget iframe's window and
ev.origin === the widget's origin. Applied to the call and to room widgets
(so one widget can't impersonate another). Verified: the spoof no longer
opens the mic; PTT/deafen from inside the call, screenshare, speaking
indicator and room widgets (capability prompt, send, live events) unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/widgets/RoomWidgetView.tsx

@@ -12,6 +12,7 @@ import { Capability, ClientWidgetApi, IRoomEvent, Widget } from 'matrix-widget-a
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { GeneralWidgetDriver } from './GeneralWidgetDriver';
 import { WidgetPermissionRequest, isWidgetUrlSafe } from './widgetUtils';
+import { restrictWidgetMessages } from '../../../plugins/widgetTransport';
 import { WidgetPermissionPrompt } from './WidgetPermissionPrompt';
 
 type PendingAsk = {
@@ -96,6 +97,9 @@ export function RoomWidgetView({ room, widget }: RoomWidgetViewProps) {
         }),
     );
     const clientApi = new ClientWidgetApi(current, iframe, driver);
+    // Only messages from this widget's own frame and origin are handled, so a
+    // widget can't impersonate another one (or the call).
+    restrictWidgetMessages(clientApi, iframe, current.origin);
     clientApi.setViewedRoomId(room.roomId);
 
     // Live room events for widgets that were allowed to see them

src/app/plugins/call/CallEmbed.ts

@@ -29,6 +29,7 @@ import { CallControl } from './CallControl';
 import { CallControlState } from './CallControlState';
 import { verifyDenoiseAssets } from './denoiseSmokeCheck';
 import { canDelegateCapability } from './utils';
+import { restrictWidgetMessages } from '../widgetTransport';
 
 // Maximum time to wait for the embedded Element Call iframe to progress from
 // initial load to a ready/joined state. If it hasn't by then, we assume the
@@ -305,6 +306,8 @@ export class CallEmbed {
 
     const callWidgetDriver: WidgetDriver = new CallWidgetDriver(mx, room.roomId);
     const call: ClientWidgetApi = new ClientWidgetApi(widget, iframe, callWidgetDriver);
+    // Only messages from the call's own frame and origin reach the widget API.
+    restrictWidgetMessages(call, iframe, widget.origin);
 
     this.mx = mx;
     this.call = call;

src/app/plugins/widgetTransport.test.ts

@@ -0,0 +1,46 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { ClientWidgetApi } from 'matrix-widget-api';
+import { isFromWidgetFrame, restrictWidgetMessages } from './widgetTransport';
+
+const frame = {} as Window;
+const other = {} as Window;
+const ORIGIN = 'https://call.example.org';
+
+test('isFromWidgetFrame: only the widget frame at its origin', () => {
+  assert.equal(isFromWidgetFrame({ source: frame, origin: ORIGIN }, frame, ORIGIN), true);
+  assert.equal(isFromWidgetFrame({ source: other, origin: ORIGIN }, frame, ORIGIN), false);
+  assert.equal(isFromWidgetFrame({ source: frame, origin: 'null' }, frame, ORIGIN), false);
+  assert.equal(
+    isFromWidgetFrame({ source: frame, origin: 'https://evil.example' }, frame, ORIGIN),
+    false,
+  );
+  assert.equal(isFromWidgetFrame({ source: null, origin: ORIGIN }, null, ORIGIN), false);
+});
+
+test('restrictWidgetMessages swaps the listener and drops foreign messages', () => {
+  const seen: unknown[] = [];
+  const original = (ev: MessageEvent) => {
+    seen.push(ev.data);
+  };
+  const transport = { handleMessage: original };
+  const api = { transport } as unknown as ClientWidgetApi;
+  const iframe = { contentWindow: frame } as HTMLIFrameElement;
+  const listeners = new Set<EventListener>([original as EventListener]);
+  const inbound = {
+    addEventListener: (_t: string, l: EventListener) => listeners.add(l),
+    removeEventListener: (_t: string, l: EventListener) => listeners.delete(l),
+  } as unknown as Window;
+
+  restrictWidgetMessages(api, iframe, ORIGIN, inbound);
+  assert.equal(listeners.has(original as EventListener), false);
+  assert.equal(listeners.size, 1);
+  assert.equal(transport.handleMessage === original, false);
+
+  const dispatch = (ev: Partial<MessageEvent>) =>
+    listeners.forEach((l) => l(ev as unknown as Event));
+  dispatch({ source: other, origin: ORIGIN, data: 'spoof' });
+  dispatch({ source: frame, origin: 'null', data: 'wrong-origin' });
+  dispatch({ source: frame, origin: ORIGIN, data: 'real' });
+  assert.deepEqual(seen, ['real']);
+});

src/app/plugins/widgetTransport.ts

@@ -0,0 +1,47 @@
+import { ClientWidgetApi } from 'matrix-widget-api';
+
+type MessageLike = Pick<MessageEvent, 'source' | 'origin'>;
+
+/**
+ * True when a message came from the widget's own frame, at the origin the
+ * widget was loaded from.
+ */
+export const isFromWidgetFrame = (
+  ev: MessageLike,
+  frameWindow: Window | null | undefined,
+  widgetOrigin: string,
+): boolean => !!frameWindow && ev.source === frameWindow && ev.origin === widgetOrigin;
+
+type InboundTransport = {
+  handleMessage: (ev: MessageEvent) => void;
+};
+
+/**
+ * matrix-widget-api's host transport accepts a message from ANY window on the
+ * page as long as it carries the widget's id (its `strictOriginCheck` only
+ * compares against the host's own origin, and is off by default). The call
+ * widget's id is fixed ('call-embed'), so any other frame (a room widget, a
+ * URL-preview embed) could post fromWidget actions as the call: e.g. a fake
+ * push-to-talk keydown turned the user's mic on.
+ *
+ * Swap the transport's listener for one that only lets through messages from
+ * this widget's iframe and origin. `stop()` removes `handleMessage`, which is
+ * the guarded one after this. Call right after `new ClientWidgetApi(...)`,
+ * which has already started the transport.
+ */
+export const restrictWidgetMessages = (
+  api: ClientWidgetApi,
+  iframe: HTMLIFrameElement,
+  widgetOrigin: string,
+  inbound: Pick<Window, 'addEventListener' | 'removeEventListener'> = window,
+): void => {
+  const transport = api.transport as unknown as InboundTransport;
+  const original = transport.handleMessage;
+  const guarded = (ev: MessageEvent) => {
+    if (!isFromWidgetFrame(ev, iframe.contentWindow, widgetOrigin)) return;
+    original(ev);
+  };
+  inbound.removeEventListener('message', original as EventListener);
+  transport.handleMessage = guarded;
+  inbound.addEventListener('message', guarded as EventListener);
+};