diff --git a/application/forms/Authentication/LoginForm.php b/application/forms/Authentication/LoginForm.php
index 537520656..42ff10ed2 100644
--- a/application/forms/Authentication/LoginForm.php
+++ b/application/forms/Authentication/LoginForm.php
@@ -3,6 +3,7 @@
 
 namespace Icinga\Forms\Authentication;
 
+use Icinga\Application\Config;
 use Icinga\Authentication\Auth;
 use Icinga\Authentication\User\ExternalBackend;
 use Icinga\User;
@@ -87,6 +88,9 @@ class LoginForm extends Form
         $authChain = $auth->getAuthChain();
         $authChain->setSkipExternalBackends(true);
         $user = new User($this->getElement('username')->getValue());
+        if (! $user->hasDomain()) {
+            $user->setDomain(Config::app()->get('authentication', 'default_domain'));
+        }
         $password = $this->getElement('password')->getValue();
         $authenticated = $authChain->authenticate($user, $password);
         if ($authenticated) {
diff --git a/library/Icinga/Authentication/Auth.php b/library/Icinga/Authentication/Auth.php
index 678f777d5..d3974113b 100644
--- a/library/Icinga/Authentication/Auth.php
+++ b/library/Icinga/Authentication/Auth.php
@@ -259,6 +259,9 @@ class Auth
         foreach ($this->getAuthChain() as $userBackend) {
             if ($userBackend instanceof ExternalBackend) {
                 if ($userBackend->authenticate($user)) {
+                    if (! $user->hasDomain()) {
+                        $user->setDomain(Config::app()->get('authentication', 'default_domain'));
+                    }
                     $this->setAuthenticated($user);
                     return true;
                 }
@@ -293,6 +296,9 @@ class Auth
             return false;
         }
         $user = new User($credentials[0]);
+        if (! $user->hasDomain()) {
+            $user->setDomain(Config::app()->get('authentication', 'default_domain'));
+        }
         $password = $credentials[1];
         if ($this->getAuthChain()->setSkipExternalBackends(true)->authenticate($user, $password)) {
             $this->setAuthenticated($user, false);