diff --git a/data/web/inc/sessions.inc.php b/data/web/inc/sessions.inc.php
index 873d3e4e7..8dc86bc04 100644
--- a/data/web/inc/sessions.inc.php
+++ b/data/web/inc/sessions.inc.php
@@ -134,12 +134,13 @@ if (isset($_POST["logout"])) {
 // Check session
 function session_check() {
   global $DEV_MODE;
+  
   if (isset($_SESSION['mailcow_cc_api']) && $_SESSION['mailcow_cc_api'] === true) {
     return true;
   }
   if (!isset($_SESSION['SESS_REMOTE_UA']) || ($_SESSION['SESS_REMOTE_UA'] != $_SERVER['HTTP_USER_AGENT'])) {
     // In development mode, allow User-Agent changes (e.g., for responsive testing in dev tools)
-    if ($DEV_MODE && isset($_SESSION['SESS_REMOTE_UA'])) {
+    if ($DEV_MODE && isset($_SESSION['SESS_REMOTE_UA']) && !empty($_SERVER['HTTP_USER_AGENT']) && strlen($_SERVER['HTTP_USER_AGENT']) < 500) {
       $_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
       return true;
     }