diff --git a/lib/extension.js b/lib/extension.js
index 54d15fa7..12289828 100755
--- a/lib/extension.js
+++ b/lib/extension.js
@@ -79,7 +79,8 @@ class Extension {
      */
     this.stack = new Stack(initializationData.data.stack, postRobot);
 
-    postRobot.on('extensionEvent', (event) => {
+    // Only the host (the window that embeds this iframe) may send events.
+    postRobot.on('extensionEvent', { window: window.parent }, (event) => {
       if (event.data.name === 'entrySave') {
         emitter.emitEvent('entrySave', [{ data: event.data.data }]);
         emitter.emitEvent('updateFields', [{ data: event.data.data }]);
diff --git a/test/hostOnlyEventsSpec.js b/test/hostOnlyEventsSpec.js
new file mode 100644
index 00000000..d9114ef1
--- /dev/null
+++ b/test/hostOnlyEventsSpec.js
@@ -0,0 +1,77 @@
+import testData from './data/testData.json';
+import Extension from '../lib/extension.js';
+
+// Security bug: extensionEvent must be accepted only from the host window
+// (window.parent), never from a sibling iframe or a popup.
+// Karma runs specs in an iframe, so window.parent is replaced with a separate
+// frame's window to prove the listener is scoped to whatever the parent is.
+describe('Extension host-only events', () => {
+  let parentDescriptor;
+  let hostFrame;
+  let siblingFrame;
+
+  const listeners = () => window.__postRobot__.requestListeners.extensionEvent;
+
+  beforeEach(() => {
+    // other specs build Extension too; post-robot allows one listener per window
+    delete window.__postRobot__.requestListeners.extensionEvent;
+    hostFrame = document.createElement('iframe');
+    siblingFrame = document.createElement('iframe');
+    document.body.appendChild(hostFrame);
+    document.body.appendChild(siblingFrame);
+    parentDescriptor = Object.getOwnPropertyDescriptor(window, 'parent');
+    Object.defineProperty(window, 'parent', { configurable: true, get: () => hostFrame.contentWindow });
+  });
+
+  afterEach(() => {
+    Object.defineProperty(window, 'parent', parentDescriptor);
+    hostFrame.remove();
+    siblingFrame.remove();
+  });
+
+  it('registers the extensionEvent listener for window.parent only', () => {
+    expect(window.parent).not.toBe(window);
+    // eslint-disable-next-line no-new
+    new Extension({ data: JSON.parse(JSON.stringify(testData)) });
+
+    expect(listeners().get(hostFrame.contentWindow)).toBeDefined();
+    expect(listeners().get(siblingFrame.contentWindow)).toBeUndefined();
+    expect(listeners().get(window.__postRobot__.WINDOW_WILDCARD)).toBeUndefined();
+  });
+
+  // Behaviour: a request-shaped message, as post-robot's own 'message' listener
+  // receives it, from the host is handled; the same message from a sibling is dropped.
+  let messageId = 0;
+  const deliver = (source, payload) => {
+    messageId += 1;
+    window.dispatchEvent(new MessageEvent('message', {
+      source,
+      origin: window.location.origin,
+      data: JSON.stringify({
+        __postRobot__: {
+          type: 'postrobot_message_request',
+          id: `host-only-spec-${messageId}`,
+          hash: `host-only-spec-${messageId}`,
+          name: 'extensionEvent',
+          sourceDomain: window.location.origin,
+          fireAndForget: true,
+          data: payload
+        }
+      })
+    }));
+  };
+
+  it('runs onSave for an entrySave from the host and ignores one from a sibling frame', (done) => {
+    const extension = new Extension({ data: JSON.parse(JSON.stringify(testData)) });
+    const saved = [];
+    extension.entry.onSave((data) => saved.push(data));
+
+    deliver(siblingFrame.contentWindow, { name: 'entrySave', data: { from: 'sibling' } });
+    deliver(hostFrame.contentWindow, { name: 'entrySave', data: { from: 'host' } });
+
+    setTimeout(() => {
+      expect(saved).toEqual([{ from: 'host' }]);
+      done();
+    }, 100);
+  });
+});