Avoid merging properties on to __proto__ objects.

lodash.js

@@ -1245,6 +1245,20 @@
     return result;
   }
 
+  /**
+   * Gets the value at `key`, unless `key` is "__proto__".
+   *
+   * @private
+   * @param {Object} object The object to query.
+   * @param {string} key The key of the property to get.
+   * @returns {*} Returns the property value.
+   */
+  function safeGet(object, key) {
+    return key == '__proto__'
+      ? undefined
+      : object[key];
+  }
+
   /**
    * Converts `set` to an array of its values.
    *
@@ -3615,7 +3629,7 @@
         }
         else {
           var newValue = customizer
-            ? customizer(object[key], srcValue, (key + ''), object, source, stack)
+            ? customizer(safeGet(object, key), srcValue, (key + ''), object, source, stack)
             : undefined;
 
           if (newValue === undefined) {
@@ -3642,8 +3656,8 @@
      *  counterparts.
      */
     function baseMergeDeep(object, source, key, srcIndex, mergeFunc, customizer, stack) {
-      var objValue = object[key],
-          srcValue = source[key],
+      var objValue = safeGet(object, key),
+          srcValue = safeGet(source, key),
           stacked = stack.get(srcValue);
 
       if (stacked) {