roundcubemail-plugins-kolab/plugins/loginfail/loginfail.php

<?php

class loginfail extends rcube_plugin
{
    public $task = 'login';

    public function init()
    {
        $this->add_hook('login_failed', [$this, 'login_failed']);
    }

    public function login_failed($args)
    {
        $rcmail = rcmail::get_instance();

        $filename = $this->home . '/loginfail.html';

        if (file_exists($filename)) {
            $html = file_get_contents($filename);
            $rcmail->output->add_footer($html);
        }
    }
}