diff -Naur a/login.php b/login.php
--- a/login.php	2018-01-06 22:18:02.000000000 +0100
+++ b/login.php	2018-01-06 23:13:50.562577137 +0100
@@ -130,7 +130,8 @@
     /* Explicitly set language in un-authenticated session. */
     $registry->setLanguage($GLOBALS['language']);
 } elseif (Horde_Util::getPost('login_post') ||
-          Horde_Util::getPost('login_button')) {
+          Horde_Util::getPost('login_button') || 
+          !empty($_SERVER['REMOTE_USER'])) {
     $select_view = Horde_Util::getPost('horde_select_view');
     if ($select_view == 'mobile_nojs') {
         $nojs = true;
@@ -144,6 +145,16 @@
         'password' => Horde_Util::getPost('horde_pass'),
         'mode' => $select_view
     );
+    $user = Horde_Util::getPost('horde_user');
+
+    /* Get auth from SSO */
+    if ($_SERVER['PHP_AUTH_PW']) {
+        $auth_params = array(
+            'password' => $_SERVER['PHP_AUTH_PW'],
+            'mode' => $select_view
+        );
+        $user = $_SERVER['REMOTE_USER'];
+    }
 
     try {
         $result = $auth->getLoginParams();
@@ -152,7 +163,7 @@
         }
     } catch (Horde_Exception $e) {}
 
-    if ($auth->authenticate(Horde_Util::getPost('horde_user'), $auth_params)) {
+    if ($auth->authenticate($user, $auth_params)) {
         Horde::log(
             sprintf(
                 'Login success for %s to %s (%s)',