diff --git a/phpBB/includes/acp/acp_styles.php b/phpBB/includes/acp/acp_styles.php
index 88850d59b3..dd5255f755 100644
--- a/phpBB/includes/acp/acp_styles.php
+++ b/phpBB/includes/acp/acp_styles.php
@@ -695,6 +695,15 @@ parse_css_file = {PARSE_CSS_FILE}
 		{
 			trigger_error($user->lang['NO_TEMPLATE'] . adm_back_link($this->u_action), E_USER_WARNING);
 		}
+		
+		if ($save_changes && !check_form_key('acp_styles'))
+		{
+			trigger_error($user->lang['FORM_INVALID'] . adm_back_link($this->u_action), E_USER_WARNING);
+		}
+		else if (!$save_changes)
+		{
+			add_form_key('acp_styles');
+		}
 
 		// save changes to the template if the user submitted any
 		if ($save_changes && $template_file)