diff --git a/.eslintrc b/.eslintrc
index 2ca6e7a..5815b7f 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -9,6 +9,7 @@
 		"eqeqeq": [2, "allow-null"],
 		"indent": [2, 2],
 		"max-depth": [2, 5],
+		"max-params": [2, 3],
 		"max-statements": [2, 29],
 		"multiline-comment-style": 0,
 		"no-continue": [1],
diff --git a/lib/extend.js b/lib/extend.js
index d437caf..31c0c5e 100644
--- a/lib/extend.js
+++ b/lib/extend.js
@@ -10,6 +10,39 @@
  * Port of jQuery.extend that actually works on node.js
  */
 var is = require('is');
+var has = require('has');
+
+var defineProperty = Object.defineProperty;
+var gOPD = Object.getOwnPropertyDescriptor;
+
+// If name is '__proto__', and Object.defineProperty is available, define __proto__ as an own property on target
+var setProperty = function setP(target, name, value) {
+  if (defineProperty && name === '__proto__') {
+    defineProperty(target, name, {
+      enumerable: true,
+      configurable: true,
+      value: value,
+      writable: true
+    });
+  } else {
+    target[name] = value;
+  }
+};
+
+// Return undefined instead of __proto__ if '__proto__' is not an own property
+var getProperty = function getP(obj, name) {
+  if (name === '__proto__') {
+    if (!has(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];
+};
 
 // eslint-disable-next-line func-style
 function extend() {
@@ -41,8 +74,8 @@ function extend() {
       }
       // 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) {
@@ -59,11 +92,11 @@ function extend() {
           }
 
           // Never move original objects, clone them
-          target[name] = extend(deep, clone, copy);
+          setProperty(target, name, extend(deep, clone, copy));
 
         // Don't bring in undefined values
         } else if (typeof copy !== 'undefined') {
-          target[name] = copy;
+          setProperty(target, name, copy);
         }
       }
     }
diff --git a/package.json b/package.json
index 8298a27..52178a9 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,8 @@
   ],
   "author": "dreamerslab <ben@dreamerslab.com>",
   "dependencies": {
-    "is": "^3.1.0"
+    "has": "^1.0.3",
+    "is": "^3.2.1"
   },
   "devDependencies": {
     "@ljharb/eslint-config": "^13.0.0",
diff --git a/test/index.js b/test/index.js
index 2386469..d57c2ff 100644
--- a/test/index.js
+++ b/test/index.js
@@ -571,3 +571,15 @@ test('deep clone; arrays are merged', function (t) {
   t.deepEqual(target, expectedTarget, 'arrays are merged');
   t.end();
 });
+
+test('__proto__ is merged as an own property', { skip: !Object.defineProperty }, 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();
+});