diff --git a/packages/studio/src/App.tsx b/packages/studio/src/App.tsx
index f2a9313e6d..4f915743f8 100644
--- a/packages/studio/src/App.tsx
+++ b/packages/studio/src/App.tsx
@@ -333,7 +333,7 @@ export function StudioApp() {
         : null,
     effectiveTimelineDuration,
   });
-  const compositionDimensions = useCompositionDimensions();
+  const compositionDimensions = useCompositionDimensions(previewIframeRef);
   const { lintModal, linting, handleLint, closeLintModal, findingsByFile } = useLintModal(
     projectId,
     refreshKey,
diff --git a/packages/studio/src/captions/components/CaptionOverlay.sender.test.tsx b/packages/studio/src/captions/components/CaptionOverlay.sender.test.tsx
new file mode 100644
index 0000000000..4f62397722
--- /dev/null
+++ b/packages/studio/src/captions/components/CaptionOverlay.sender.test.tsx
@@ -0,0 +1,47 @@
+// @vitest-environment jsdom
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { expect, it, vi } from "vitest";
+import { CaptionOverlay } from "./CaptionOverlay";
+import { useCaptionStore } from "../store";
+
+Reflect.set(globalThis, "IS_REACT_ACT_ENVIRONMENT", true);
+
+it("only schedules caption layout for messages from its preview", () => {
+  const host = document.createElement("div");
+  const preview = document.createElement("iframe");
+  const foreign = document.createElement("iframe");
+  document.body.append(host, preview, foreign);
+  const root = createRoot(host);
+  const schedule = vi.fn(() => 1);
+  vi.stubGlobal("requestAnimationFrame", schedule);
+  vi.stubGlobal("cancelAnimationFrame", vi.fn());
+  vi.stubGlobal(
+    "ResizeObserver",
+    class {
+      observe() {}
+      disconnect() {}
+    },
+  );
+  useCaptionStore.getState().setEditMode(true);
+  const send = (source: MessageEventSource | null) =>
+    act(() => {
+      window.dispatchEvent(new MessageEvent("message", { source, data: { source: "hf-preview" } }));
+    });
+  try {
+    act(() => root.render(<CaptionOverlay iframeRef={{ current: preview }} />));
+    schedule.mockClear();
+    send(foreign.contentWindow);
+    send(null);
+    expect(schedule).not.toHaveBeenCalled();
+    send(preview.contentWindow);
+    expect(schedule).toHaveBeenCalledOnce();
+  } finally {
+    act(() => root.unmount());
+    useCaptionStore.getState().reset();
+    host.remove();
+    preview.remove();
+    foreign.remove();
+    vi.unstubAllGlobals();
+  }
+});
diff --git a/packages/studio/src/captions/components/CaptionOverlay.tsx b/packages/studio/src/captions/components/CaptionOverlay.tsx
index 5681b25a26..cf1da7af16 100644
--- a/packages/studio/src/captions/components/CaptionOverlay.tsx
+++ b/packages/studio/src/captions/components/CaptionOverlay.tsx
@@ -116,6 +116,7 @@ export const CaptionOverlay = memo(function CaptionOverlay({ iframeRef }: Captio
     // messages, element resize, window resize.
     const unsubPlayer = usePlayerStore.subscribe(scheduleTick);
     const handleMessage = (e: MessageEvent) => {
+      if (!e.source || e.source !== iframeRef.current?.contentWindow) return;
       const data = e.data;
       if (data?.source === "hf-preview") scheduleTick();
     };
diff --git a/packages/studio/src/components/nle/PreviewOverlays.tsx b/packages/studio/src/components/nle/PreviewOverlays.tsx
index 5bbbbcd6dd..05c8a06ca6 100644
--- a/packages/studio/src/components/nle/PreviewOverlays.tsx
+++ b/packages/studio/src/components/nle/PreviewOverlays.tsx
@@ -139,7 +139,7 @@ export function PreviewOverlays({
 }: PreviewOverlaysProps) {
   const { activeCompPath, previewIframeRef } = useStudioShellContext();
   const { captionEditMode, compositionLoading, isPlaying } = useStudioPlaybackContext();
-  const compositionDimensions = useCompositionDimensions();
+  const compositionDimensions = useCompositionDimensions(previewIframeRef);
 
   // Caption edit mode is entered automatically when captions are detected;
   // these give the author an explicit way OUT (and back in). Without them the
diff --git a/packages/studio/src/hooks/useCaptionDetection.sender.test.tsx b/packages/studio/src/hooks/useCaptionDetection.sender.test.tsx
new file mode 100644
index 0000000000..36ef06c446
--- /dev/null
+++ b/packages/studio/src/hooks/useCaptionDetection.sender.test.tsx
@@ -0,0 +1,54 @@
+// @vitest-environment jsdom
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { expect, it, vi } from "vitest";
+import { useCaptionDetection } from "./useCaptionDetection";
+
+Reflect.set(globalThis, "IS_REACT_ACT_ENVIRONMENT", true);
+
+it("does not let a foreign window trigger caption activation", () => {
+  const host = document.createElement("div");
+  const preview = document.createElement("iframe");
+  const foreign = document.createElement("iframe");
+  document.body.append(host, preview, foreign);
+  const root = createRoot(host);
+  const fetchMock = vi.fn(() => new Promise<Response>(() => {}));
+  vi.stubGlobal("fetch", fetchMock);
+  const params = {
+    projectId: "demo",
+    activeCompPath: "captions.html",
+    compIdToSrc: new Map<string, string>(),
+    captionEditMode: false,
+    captionHasSelection: false,
+    previewIframeRef: { current: preview },
+    captionSync: { save: vi.fn(), loadOverrides: vi.fn(async () => {}) },
+    setRightCollapsed: vi.fn(),
+  };
+  function Harness() {
+    useCaptionDetection(params);
+    return null;
+  }
+  const send = (source: MessageEventSource | null) =>
+    act(() => {
+      window.dispatchEvent(
+        new MessageEvent("message", { source, data: { source: "hf-preview", type: "state" } }),
+      );
+    });
+  try {
+    act(() => root.render(<Harness />));
+    const group = preview.contentDocument!.createElement("div");
+    group.className = "caption-group";
+    preview.contentDocument!.body.append(group);
+    send(foreign.contentWindow);
+    send(null);
+    expect(fetchMock).not.toHaveBeenCalled();
+    send(preview.contentWindow);
+    expect(fetchMock).toHaveBeenCalledOnce();
+  } finally {
+    act(() => root.unmount());
+    host.remove();
+    preview.remove();
+    foreign.remove();
+    vi.unstubAllGlobals();
+  }
+});
diff --git a/packages/studio/src/hooks/useCaptionDetection.ts b/packages/studio/src/hooks/useCaptionDetection.ts
index e7fce63c47..b7554b174f 100644
--- a/packages/studio/src/hooks/useCaptionDetection.ts
+++ b/packages/studio/src/hooks/useCaptionDetection.ts
@@ -131,6 +131,7 @@ export function useCaptionDetection({
     };
 
     const handleMessage = (e: MessageEvent) => {
+      if (!e.source || e.source !== previewIframeRef.current?.contentWindow) return;
       const data = e.data;
       if (data?.source === "hf-preview" && (data?.type === "state" || data?.type === "timeline")) {
         if (!acceptStudioRuntimeMessage(data)) return;
diff --git a/packages/studio/src/hooks/useCompositionDimensions.test.tsx b/packages/studio/src/hooks/useCompositionDimensions.test.tsx
new file mode 100644
index 0000000000..ec02a8f6c5
--- /dev/null
+++ b/packages/studio/src/hooks/useCompositionDimensions.test.tsx
@@ -0,0 +1,47 @@
+// @vitest-environment jsdom
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { expect, it } from "vitest";
+import { useCompositionDimensions } from "./useCompositionDimensions";
+
+Reflect.set(globalThis, "IS_REACT_ACT_ENVIRONMENT", true);
+
+it("accepts only the current preview frame and rejects foreign or missing senders", () => {
+  const host = document.createElement("div");
+  const preview = document.createElement("iframe");
+  const foreign = document.createElement("iframe");
+  document.body.append(host, preview, foreign);
+  const ref = { current: preview };
+  const root = createRoot(host);
+  function Harness() {
+    return JSON.stringify(useCompositionDimensions(ref));
+  }
+  const send = (source: MessageEventSource | null, width: number) =>
+    act(() => {
+      window.dispatchEvent(
+        new MessageEvent("message", {
+          source,
+          origin: window.location.origin,
+          data: { source: "hf-preview", type: "stage-size", width, height: 100 },
+        }),
+      );
+    });
+  try {
+    act(() => root.render(<Harness />));
+    send(foreign.contentWindow, 300);
+    send(null, 400);
+    expect(host.textContent).toBe("null");
+    send(preview.contentWindow, 200);
+    expect(host.textContent).toBe('{"width":200,"height":100}');
+    ref.current = foreign;
+    send(preview.contentWindow, 500);
+    expect(host.textContent).toBe('{"width":200,"height":100}');
+    send(foreign.contentWindow, 600);
+    expect(host.textContent).toBe('{"width":600,"height":100}');
+  } finally {
+    act(() => root.unmount());
+    host.remove();
+    preview.remove();
+    foreign.remove();
+  }
+});
diff --git a/packages/studio/src/hooks/useCompositionDimensions.ts b/packages/studio/src/hooks/useCompositionDimensions.ts
index 49a5371496..da83c2a2ac 100644
--- a/packages/studio/src/hooks/useCompositionDimensions.ts
+++ b/packages/studio/src/hooks/useCompositionDimensions.ts
@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { useState, type RefObject } from "react";
 import { useMountEffect } from "./useMountEffect";
 import type { CompositionDimensions } from "../components/renders/RenderQueue";
 import { acceptStudioRuntimeMessage } from "../player/lib/runtimeProtocol";
@@ -29,13 +29,14 @@ function readPositiveDimensions(width: unknown, height: unknown): CompositionDim
   return { width: parsedWidth, height: parsedHeight };
 }
 
-export function useCompositionDimensions() {
+export function useCompositionDimensions(iframeRef: RefObject<HTMLIFrameElement | null>) {
   const [compositionDimensions, setCompositionDimensions] = useState<CompositionDimensions | null>(
     null,
   );
 
   useMountEffect(() => {
     const handleMessage = (e: MessageEvent) => {
+      if (!e.source || e.source !== iframeRef.current?.contentWindow) return;
       const dimensions = readCompositionSizeMessage(e.data);
       if (!dimensions) return;
       setCompositionDimensions((prev) =>