Prevent merging __proto__ property

index.js

@@ -2,6 +2,8 @@
 
 var hasOwn = Object.prototype.hasOwnProperty;
 var toStr = Object.prototype.toString;
+var defineProperty = Object.defineProperty;
+var gOPD = Object.getOwnPropertyDescriptor;
 
 var isArray = function isArray(arr) {
 	if (typeof Array.isArray === 'function') {
@@ -31,6 +33,35 @@ var isPlainObject = function isPlainObject(obj) {
 	return typeof key === 'undefined' || hasOwn.call(obj, key);
 };
 
+// If name is '__proto__', and Object.defineProperty is available, define __proto__ as an own property on target
+var setProperty = function setProperty(target, options) {
+	if (defineProperty && options.name === '__proto__') {
+		defineProperty(target, options.name, {
+			enumerable: true,
+			configurable: true,
+			value: options.newValue,
+			writable: true
+		});
+	} else {
+		target[options.name] = options.newValue;
+	}
+};
+
+// Return undefined instead of __proto__ if '__proto__' is not an own property
+var getProperty = function getProperty(obj, name) {
+	if (name === '__proto__') {
+		if (!hasOwn.call(obj, name)) {
+			return void 0;
+		} else if (gOPD) {
+			// In early versions of node, obj['__proto__'] is buggy when obj has
+			// __proto__ as an own property. Object.getOwnPropertyDescriptor() works.
+			return gOPD(obj, name).value;
+		}
+	}
+
+	return obj[name];
+};
+
 module.exports = function extend() {
 	var options, name, src, copy, copyIsArray, clone;
 	var target = arguments[0];
@@ -55,8 +86,8 @@ module.exports = function extend() {
 		if (options != null) {
 			// Extend the base object
 			for (name in options) {
-				src = target[name];
-				copy = options[name];
+				src = getProperty(target, name);
+				copy = getProperty(options, name);
 
 				// Prevent never-ending loop
 				if (target !== copy) {
@@ -70,11 +101,11 @@ module.exports = function extend() {
 						}
 
 						// Never move original objects, clone them
-						target[name] = extend(deep, clone, copy);
+						setProperty(target, { name: name, newValue: extend(deep, clone, copy) });
 
 					// Don't bring in undefined values
 					} else if (typeof copy !== 'undefined') {
-						target[name] = copy;
+						setProperty(target, { name: name, newValue: copy });
 					}
 				}
 			}

test/index.js

@@ -626,3 +626,15 @@ test('non-object target', function (t) {
 
 	t.end();
 });
+
+test('__proto__ is merged as an own property', function (t) {
+	var malicious = { fred: 1 };
+	Object.defineProperty(malicious, '__proto__', { value: { george: 1 }, enumerable: true });
+	var target = {};
+	extend(true, target, malicious);
+	t.notOk(target.george);
+	t.ok(Object.prototype.hasOwnProperty.call(target, '__proto__'));
+	t.deepEqual(Object.getOwnPropertyDescriptor(target, '__proto__').value, { george: 1 });
+
+	t.end();
+});