yii\web\Request::cookieValidationKey is now automatically generated by the installation script for the basic and advanced application templates

composer.json

@@ -31,7 +31,8 @@
     },
     "scripts": {
         "post-create-project-cmd": [
-			"yii\\composer\\Installer::setPermission"
+            "yii\\composer\\Installer::setPermission",
+            "yii\\composer\\Installer::generateCookieValidationKey"
         ]
     },
     "config": {
@@ -44,6 +45,9 @@
         ],
         "executable": [
             "yii"
+        ],
+        "config": [
+            "config/web.php"
         ]
     }
 }

config/web.php

@@ -7,6 +7,10 @@ $config = [
     'basePath' => dirname(__DIR__),
     'bootstrap' => ['log'],
     'components' => [
+        'request' => [
+            // a secret key used to validate cookies. You may modify this key with your own one.
+            'cookieValidationKey' => '',
+        ],
         'cache' => [
             'class' => 'yii\caching\FileCache',
         ],