diff --git a/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.cpp b/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.cpp
index 764716621f..868bded595 100644
--- a/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.cpp
+++ b/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.cpp
@@ -55,6 +55,16 @@ Vector<CrossOriginProperty> cross_origin_properties(Variant<LocationObject const
         });
 }
 
+// https://html.spec.whatwg.org/multipage/browsers.html#cross-origin-accessible-window-property-name
+bool is_cross_origin_accessible_window_property_name(JS::PropertyKey const& property_key)
+{
+    // A JavaScript property name P is a cross-origin accessible window property name if it is "window", "self", "location", "close", "closed", "focus", "blur", "frames", "length", "top", "opener", "parent", "postMessage", or an array index property name.
+    static Array<FlyString, 13> property_names {
+        "window"sv, "self"sv, "location"sv, "close"sv, "closed"sv, "focus"sv, "blur"sv, "frames"sv, "length"sv, "top"sv, "opener"sv, "parent"sv, "postMessage"sv
+    };
+    return (property_key.is_string() && any_of(property_names, [&](auto const& name) { return property_key.as_string() == name; })) || property_key.is_number();
+}
+
 // 7.2.3.2 CrossOriginPropertyFallback ( P ), https://html.spec.whatwg.org/multipage/browsers.html#crossoriginpropertyfallback-(-p-)
 JS::ThrowCompletionOr<JS::PropertyDescriptor> cross_origin_property_fallback(JS::GlobalObject& global_object, JS::PropertyKey const& property_key)
 {
diff --git a/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.h b/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.h
index fab6a14993..8c0081d5af 100644
--- a/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.h
+++ b/Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.h
@@ -28,6 +28,7 @@ struct CrossOriginKey {
 using CrossOriginPropertyDescriptorMap = HashMap<CrossOriginKey, JS::PropertyDescriptor>;
 
 Vector<CrossOriginProperty> cross_origin_properties(Variant<LocationObject const*, WindowObject const*> const&);
+bool is_cross_origin_accessible_window_property_name(JS::PropertyKey const&);
 JS::ThrowCompletionOr<JS::PropertyDescriptor> cross_origin_property_fallback(JS::GlobalObject&, JS::PropertyKey const&);
 bool is_platform_object_same_origin(JS::Object const&);
 Optional<JS::PropertyDescriptor> cross_origin_get_own_property_helper(Variant<LocationObject*, WindowObject*> const&, JS::PropertyKey const&);