diff --git a/src/lib/iframe.coffee b/src/lib/iframe.coffee
index d14199a..a9476f0 100644
--- a/src/lib/iframe.coffee
+++ b/src/lib/iframe.coffee
@@ -130,6 +130,9 @@ class IframeRuntime extends Base
     ), w.location.href
 
   onMessage: (message) =>
+    if message.source and message.source isnt @iframe.contentWindow
+      # Message from unrelated source
+      return
     if typeof message.data is 'string'
       data = JSON.parse message.data
     else
diff --git a/src/lib/opener.coffee b/src/lib/opener.coffee
index d353506..2c18c9a 100644
--- a/src/lib/opener.coffee
+++ b/src/lib/opener.coffee
@@ -83,6 +83,9 @@ class OpenerRuntime extends Base
     ), '*'
 
   onMessage: (message) =>
+    if message.source and message.source isnt @iframe.contentWindow
+      # Message from unrelated source
+      return
     if typeof message.data is 'string'
       data = JSON.parse message.data
     else