feat(lotus): work when served from its own origin (cinny #43)

Two things tied EC to the host's origin:

- Widget message check: matrix-widget-api's strictOriginCheck compares
  ev.origin with THIS frame's origin, so on call.chat.lotusguild.org every
  message from chat.lotusguild.org would be dropped and calls would not
  start. restrictToHost() instead requires ev.source === window.parent and
  ev.origin === parentUrl's origin. Same-origin deployments keep working
  (the host origin is our own origin there), and it is stricter than
  before: the sender must also be our parent window.
- Soundboard: the host's blob: clip URL is origin-bound. io.lotus.inject_audio
  now accepts the clip's bytes (`audio`, ArrayBuffer, <= 8 MiB) and prefers
  them over `url`; hosts that only send `url` are unchanged.

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

src/lotus/lotusAudioInject.test.ts

@@ -10,7 +10,11 @@ import { afterEach, beforeEach, expect, test, vi } from "vitest";
 import { of } from "rxjs";
 
 import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
-import { startLotusAudioInject } from "./lotusAudioInject";
+import {
+  MAX_INJECT_BYTES,
+  parseInjectSource,
+  startLotusAudioInject,
+} from "./lotusAudioInject";
 import { LotusWidgetActions } from "./lotusActions";
 
 const lazyActions = new EventEmitter();
@@ -223,3 +227,34 @@ test("#14: the shared context stays open while another instance is still active"
   stopB();
   expect(ctx.close).toHaveBeenCalledTimes(1);
 });
+
+// [cinny #43] Cross-origin hosts send the clip bytes instead of a blob: URL.
+test("parseInjectSource prefers the clip bytes over the url", () => {
+  const audio = new ArrayBuffer(16);
+  expect(parseInjectSource({ audio, url: "https://x.example/a.ogg" })).toBe(
+    audio,
+  );
+});
+
+test("parseInjectSource falls back to a safe url", () => {
+  expect(parseInjectSource({ url: "https://x.example/a.ogg" })).toBe(
+    "https://x.example/a.ogg",
+  );
+  expect(
+    parseInjectSource({
+      audio: new ArrayBuffer(0),
+      url: "https://x.example/a.ogg",
+    }),
+  ).toBe("https://x.example/a.ogg");
+  expect(parseInjectSource({ url: "javascript" + ":alert(1)" })).toBeNull();
+  expect(parseInjectSource({})).toBeNull();
+  expect(parseInjectSource(undefined)).toBeNull();
+});
+
+test("parseInjectSource rejects oversized or non-buffer audio", () => {
+  expect(
+    parseInjectSource({ audio: new ArrayBuffer(MAX_INJECT_BYTES + 1) }),
+  ).toBeNull();
+  expect(parseInjectSource({ audio: "not bytes" })).toBeNull();
+  expect(parseInjectSource({ audio: new Uint8Array(4) })).toBeNull();
+});

src/lotus/lotusAudioInject.ts

@@ -52,8 +52,10 @@ function acquireSharedAudio(): {
  * that was impossible against the prebuilt EC bundle (LiveKit's
  * LocalParticipant lived in EC's module scope).
  *
- * Action data: `{ url: string, volume?: number }`. `url` must be an https/blob
- * URL (the host resolves mxc → media URL).
+ * Action data: `{ url?: string, audio?: ArrayBuffer, volume?: number }`.
+ * `audio` is the clip's bytes (cinny #43: a host `blob:` URL can't be fetched
+ * once EC is served from its own origin, so the host sends the bytes and they
+ * win); otherwise `url` must be an https/blob URL (older hosts).
  *
  * No effect unless the host sends the action. Returns a teardown function that
  * also aborts any clip still playing.
@@ -88,12 +90,12 @@ export function startLotusAudioInject(vm: CallViewModel): () => void {
       return;
     }
     const data = ev.detail.data as
-      | { url?: unknown; volume?: unknown }
+      | { url?: unknown; audio?: unknown; volume?: unknown }
       | undefined;
-    const url = typeof data?.url === "string" ? safeMediaUrl(data.url) : null;
-    if (!url) {
+    const clip = parseInjectSource(data);
+    if (!clip) {
       w.api.transport.reply(ev.detail, {});
-      logger.warn("[lotus] inject_audio: missing/invalid url");
+      logger.warn("[lotus] inject_audio: missing/invalid url or audio");
       return;
     }
     const volume =
@@ -115,7 +117,7 @@ export function startLotusAudioInject(vm: CallViewModel): () => void {
     }
 
     w.api.transport.reply(ev.detail, {});
-    void playInjectedClip(url, volume, rooms, activeClips).catch((e) =>
+    void playInjectedClip(clip, volume, rooms, activeClips).catch((e) =>
       logger.warn("[lotus] inject_audio failed", e),
     );
   };
@@ -144,6 +146,27 @@ export function startLotusAudioInject(vm: CallViewModel): () => void {
   };
 }
 
+/** Largest clip accepted as bytes (the host's soundboard clips are small). */
+export const MAX_INJECT_BYTES = 8 * 1024 * 1024;
+
+/**
+ * The clip to play: the bytes the host sent (`audio`), or else a fetchable
+ * `url`. Exported for tests.
+ */
+export function parseInjectSource(
+  data: { url?: unknown; audio?: unknown } | undefined,
+): string | ArrayBuffer | null {
+  const audio = data?.audio;
+  if (
+    audio instanceof ArrayBuffer &&
+    audio.byteLength > 0 &&
+    audio.byteLength <= MAX_INJECT_BYTES
+  ) {
+    return audio;
+  }
+  return typeof data?.url === "string" ? safeMediaUrl(data.url) : null;
+}
+
 /** Only allow fetchable media URLs; never same-origin credentialed GETs etc. */
 function safeMediaUrl(raw: string): string | null {
   try {
@@ -155,7 +178,7 @@ function safeMediaUrl(raw: string): string | null {
 }
 
 async function playInjectedClip(
-  url: string,
+  clip: string | ArrayBuffer,
   volume: number,
   rooms: LivekitRoom[],
   activeClips: Set<() => void>,
@@ -188,25 +211,30 @@ async function playInjectedClip(
   };
   activeClips.add(placeholder);
 
-  let resp: Response;
-  try {
-    resp = await fetch(url, {
-      credentials: "omit",
-      mode: "cors",
-      signal: controller.signal,
-    });
-  } catch (e) {
-    // Superseded by a newer clip mid-fetch — expected, not a failure.
+  let arrayBuffer: ArrayBuffer;
+  if (typeof clip !== "string") {
+    arrayBuffer = clip;
+  } else {
+    let resp: Response;
+    try {
+      resp = await fetch(clip, {
+        credentials: "omit",
+        mode: "cors",
+        signal: controller.signal,
+      });
+    } catch (e) {
+      // Superseded by a newer clip mid-fetch — expected, not a failure.
+      if (aborted) return;
+      throw e;
+    }
+    if (aborted) return;
+    if (!resp.ok) {
+      activeClips.delete(placeholder);
+      throw new Error(`fetch ${clip} -> ${resp.status}`);
+    }
+    arrayBuffer = await resp.arrayBuffer();
     if (aborted) return;
-    throw e;
   }
-  if (aborted) return;
-  if (!resp.ok) {
-    activeClips.delete(placeholder);
-    throw new Error(`fetch ${url} -> ${resp.status}`);
-  }
-  const arrayBuffer = await resp.arrayBuffer();
-  if (aborted) return;
 
   // [lotus] Reuse the shared module-level context/destination (#14) rather
   // than `new AudioContext()` per clip — see the declaration above.

src/lotus/lotusWidgetOrigin.test.ts

@@ -0,0 +1,65 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, it } from "vitest";
+
+import { isFromHost, restrictToHost } from "./lotusWidgetOrigin";
+
+const parent = {} as Window;
+const other = {} as Window;
+const HOST = "https://chat.example.org";
+
+describe("isFromHost", () => {
+  it("accepts only the parent window at the host origin", () => {
+    expect(isFromHost({ source: parent, origin: HOST }, parent, HOST)).toBe(
+      true,
+    );
+    expect(isFromHost({ source: other, origin: HOST }, parent, HOST)).toBe(
+      false,
+    );
+    expect(
+      isFromHost(
+        { source: parent, origin: "https://evil.example" },
+        parent,
+        HOST,
+      ),
+    ).toBe(false);
+    expect(isFromHost({ source: parent, origin: "null" }, parent, HOST)).toBe(
+      false,
+    );
+  });
+});
+
+describe("restrictToHost", () => {
+  it("replaces the listener and drops foreign messages", () => {
+    const seen: unknown[] = [];
+    const original = (ev: MessageEvent): void => {
+      seen.push(ev.data);
+    };
+    const transport = { handleMessage: original };
+    const listeners = new Set<EventListener>([original as EventListener]);
+    const inbound = {
+      addEventListener: (_t: string, l: EventListener): void => {
+        listeners.add(l);
+      },
+      removeEventListener: (_t: string, l: EventListener): void => {
+        listeners.delete(l);
+      },
+    } as unknown as Window;
+
+    restrictToHost(transport, HOST, parent, inbound);
+    expect(listeners.has(original as EventListener)).toBe(false);
+    expect(listeners.size).toBe(1);
+
+    const dispatch = (ev: Partial<MessageEvent>): void =>
+      listeners.forEach((l) => l(ev as unknown as Event));
+    dispatch({ source: other, origin: HOST, data: "spoof" });
+    dispatch({ source: parent, origin: "https://evil.example", data: "bad" });
+    dispatch({ source: parent, origin: HOST, data: "real" });
+    expect(seen).toEqual(["real"]);
+  });
+});

src/lotus/lotusWidgetOrigin.ts

@@ -0,0 +1,53 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+type MessageLike = Pick<MessageEvent, "source" | "origin">;
+
+/** True when a message came from our parent window, at the host's origin. */
+export function isFromHost(
+  ev: MessageLike,
+  parentWindow: Window,
+  parentOrigin: string,
+): boolean {
+  return ev.source === parentWindow && ev.origin === parentOrigin;
+}
+
+interface InboundTransport {
+  handleMessage: (ev: MessageEvent) => void;
+}
+
+/**
+ * [cinny #43] Only handle widget messages sent by the host (our parent window)
+ * from the origin it was loaded at (`parentUrl`).
+ *
+ * matrix-widget-api's `strictOriginCheck` compares `ev.origin` with this
+ * frame's OWN origin, which only works while EC is served from the host's
+ * origin; served from its own origin (e.g. call.chat.lotusguild.org) every
+ * host message would be dropped. Turning the check off instead would let any
+ * window post toWidget actions (including io.lotus.*). This compares against
+ * the host's origin and also requires the sender to be our parent, so it is
+ * correct same-origin and cross-origin alike.
+ *
+ * Swaps the transport's `message` listener; safe whether or not the transport
+ * has started (`stop()` removes `handleMessage`, the guarded one after this).
+ */
+export function restrictToHost(
+  transport: unknown,
+  parentOrigin: string,
+  parentWindow: Window = window.parent,
+  inbound: Pick<Window, "addEventListener" | "removeEventListener"> = window,
+): void {
+  const t = transport as InboundTransport;
+  const original = t.handleMessage;
+  const guarded = (ev: MessageEvent): void => {
+    if (!isFromHost(ev, parentWindow, parentOrigin)) return;
+    original(ev);
+  };
+  inbound.removeEventListener("message", original as EventListener);
+  t.handleMessage = guarded;
+  inbound.addEventListener("message", guarded as EventListener);
+}

src/widget.ts

@@ -24,6 +24,7 @@ import { Config } from "./config/Config";
 import { seedSettingsFromConfig } from "./settings/settings";
 import { ElementCallReactionEventType } from "./reactions";
 import { LOTUS_TO_WIDGET_ACTIONS } from "./lotus/lotusActions";
+import { restrictToHost } from "./lotus/lotusWidgetOrigin";
 
 // Subset of the actions in element-web
 export enum ElementWidgetActions {
@@ -94,16 +95,11 @@ export const initializeWidget = (
       const parentOrigin = new URL(parentUrl).origin;
       logger.info("Widget API is available");
       const api = new WidgetApi(widgetId, parentOrigin);
-      // [lotus] matrix-widget-api's PostmessageTransport defaults
-      // strictOriginCheck to false, which would let any frame holding a
-      // handle to our window post toWidget actions (including the
-      // io.lotus.* actions below). The Lotus deployment serves EC
-      // same-origin with the host (cinny loads /public/element-call/index.html),
-      // so globalThis.origin === parentOrigin and this check passes safely.
-      // A cross-origin deployment would need to compare ev.origin to
-      // parentOrigin instead, since strictOriginCheck compares against
-      // globalThis.origin.
-      api.transport.strictOriginCheck = true;
+      // [lotus] Only messages from the host (our parent, at parentUrl's
+      // origin) reach the widget API. matrix-widget-api's own
+      // strictOriginCheck compares against THIS frame's origin, so it only
+      // worked while EC was served from the host's origin (cinny #43).
+      restrictToHost(api.transport, parentOrigin);
       api.requestCapability(MatrixCapabilities.AlwaysOnScreen);
       api.requestCapability(MatrixCapabilities.MSC4039DownloadFile);