diff --git a/atom/browser/lib/guest-window-manager.coffee b/atom/browser/lib/guest-window-manager.coffee
index 9ed75c225f54..1e9ee13c71d7 100644
--- a/atom/browser/lib/guest-window-manager.coffee
+++ b/atom/browser/lib/guest-window-manager.coffee
@@ -74,23 +74,19 @@ ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_METHOD', (event, guestId, met
   BrowserWindow.fromId(guestId)?[method] args...
 
 ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (event, guestId, message, targetOrigin, sourceOrigin) ->
+  sourceId = BrowserWindow.fromWebContents(event.sender)?.id
+  return unless sourceId?
+
   guestContents = BrowserWindow.fromId(guestId)?.webContents
   if guestContents?.getURL().indexOf(targetOrigin) is 0 or targetOrigin is '*'
-    guestContents?.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', guestId, message, sourceOrigin
-
-ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', (event, guestId, message, targetOrigin, sourceOrigin) ->
-  embedder = v8Util.getHiddenValue event.sender, 'embedder'
-  if embedder?.getURL().indexOf(targetOrigin) is 0 or targetOrigin is '*'
-    embedder?.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', guestId, message, sourceOrigin
+    guestContents?.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', sourceId, message, sourceOrigin
 
 ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WEB_CONTENTS_METHOD', (event, guestId, method, args...) ->
   BrowserWindow.fromId(guestId)?.webContents?[method] args...
 
-ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_GUEST_ID', (event) ->
+ipcMain.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_OPENER_ID', (event) ->
   embedder = v8Util.getHiddenValue event.sender, 'embedder'
+  openerId = null
   if embedder?
-    guest = BrowserWindow.fromWebContents event.sender
-    if guest?
-      event.returnValue = guest.id
-      return
-  event.returnValue = null
+    openerId = BrowserWindow.fromWebContents(embedder)?.id
+  event.returnValue = openerId
diff --git a/atom/renderer/lib/override.coffee b/atom/renderer/lib/override.coffee
index 333acc82739d..ba15c7b3f61a 100644
--- a/atom/renderer/lib/override.coffee
+++ b/atom/renderer/lib/override.coffee
@@ -91,21 +91,17 @@ window.confirm = (message, title='') ->
 window.prompt = ->
   throw new Error('prompt() is and will not be supported.')
 
-# Implement window.postMessage if current window is a guest window.
-guestId = ipcRenderer.sendSync 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_GUEST_ID'
-if guestId?
-  window.opener =
-    postMessage: (message, targetOrigin='*') ->
-      ipcRenderer.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', guestId, message, targetOrigin, location.origin
+openerId = ipcRenderer.sendSync 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_OPENER_ID'
+window.opener = BrowserWindowProxy.getOrCreate(openerId) if openerId?
 
-ipcRenderer.on 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', (event, guestId, message, sourceOrigin) ->
+ipcRenderer.on 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', (event, sourceId, message, sourceOrigin) ->
   # Manually dispatch event instead of using postMessage because we also need to
   # set event.source.
   event = document.createEvent 'Event'
   event.initEvent 'message', false, false
   event.data = message
   event.origin = sourceOrigin
-  event.source = BrowserWindowProxy.getOrCreate(guestId)
+  event.source = BrowserWindowProxy.getOrCreate(sourceId)
   window.dispatchEvent event
 
 # Forward history operations to browser.
diff --git a/spec/chromium-spec.coffee b/spec/chromium-spec.coffee
index 2cc2237385be..c456a6919b47 100644
--- a/spec/chromium-spec.coffee
+++ b/spec/chromium-spec.coffee
@@ -116,11 +116,16 @@ describe 'chromium feature', ->
       b = window.open url, '', 'show=no'
 
   describe 'window.postMessage', ->
-    it 'sets the origin correctly', (done) ->
+    it 'sets the source and origin correctly', (done) ->
+      sourceId = remote.getCurrentWindow().id
       listener = (event) ->
         window.removeEventListener 'message', listener
         b.close()
-        assert.equal event.data, 'file://testing'
+        message = JSON.parse(event.data)
+        assert.equal message.data, 'testing'
+        assert.equal message.origin, 'file://'
+        assert.equal message.sourceEqualsOpener, true
+        assert.equal message.sourceId, sourceId
         assert.equal event.origin, 'file://'
         done()
       window.addEventListener 'message', listener
diff --git a/spec/fixtures/pages/window-open-postMessage.html b/spec/fixtures/pages/window-open-postMessage.html
index e547fa2a6090..0e7e4d8cce0c 100644
--- a/spec/fixtures/pages/window-open-postMessage.html
+++ b/spec/fixtures/pages/window-open-postMessage.html
@@ -2,7 +2,12 @@
 <body>
 <script type="text/javascript" charset="utf-8">
   window.addEventListener('message', function (e) {
-    window.opener.postMessage(e.origin + e.data, '*');
+    window.opener.postMessage(JSON.stringify({
+      origin: e.origin,
+      data: e.data,
+      sourceEqualsOpener: e.source === window.opener,
+      sourceId: e.source.guestId
+    }), '*');
   });
 </script>
 </body>