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<?php

@unlink(__FILE__);

// Validate if the request is from Softaculous
if($_REQUEST['pass'] != '[[autopass]]'){
	die("Unauthorized Access");
}

define('_JEXEC', 1);

if (file_exists(__DIR__ . '/defines.php'))
{
	include_once __DIR__ . '/defines.php';
}

if (!defined('_JDEFINES'))
{
	define('JPATH_BASE', __DIR__ . DIRECTORY_SEPARATOR . 'administrator' );
	require_once JPATH_BASE . '/includes/defines.php';
}

require_once JPATH_BASE . '/includes/framework.php';
require_once JPATH_BASE . '/includes/helper.php';
require_once JPATH_BASE . '/includes/subtoolbar.php';

$app = JFactory::getApplication('administrator');
	
// Check for a cookie if user is not logged in
if(JFactory::getUser()->get('guest')){
	
	$signon_user = '[[signon_username]]';
	
	$options = array('action' => 'core.login.admin');

$instance = Joomla\CMS\User\User::getInstance();

$db = JFactory::getDbo();
	
	if(!empty($signon_user) && !preg_match('/^\[\[(.*?)\]\]$/is', $signon_user)){
		$id = (int) Joomla\CMS\User\UserHelper::getUserId($signon_user);
	}else{
		$query = $db->getQuery(true);
		$query->select($db->quoteName('user_id'))
			->from($db->quoteName('#__user_usergroup_map'))
			->where($db->quoteName('group_id') . ' = 8' );
		$db->setQuery($query);
		$users = $db->loadColumn();
		$id = min($users);
	}
	
	if (!$id)
	{
		$app->redirect('administrator/index.php');
		exit();
	}

$instance->load($id);

// If the user is blocked, redirect with an error
	if ($instance->block == 1)
	{
		$app->redirect('administrator/index.php');
		exit();
	}

$result = $instance->authorise($options['action']);

// Mark the user as logged in
	$instance->guest = 0;

$session = Joomla\CMS\Factory::getSession();

// Grab the current session ID
	$oldSessionId = $session->getId();
			
	// Fork the session
	$session->fork();

$session->set('user', $instance);
			
	// Ensure the new session's metadata is written to the database
	$app->checkSession();

// Purge the old session
	$query = $db->getQuery(true)
		->delete('#__session')
		->where($db->quoteName('session_id') . ' = ' . $db->quoteBinary($oldSessionId));

$db->setQuery($query)->execute();

$instance->setLastVisit();

}

$app->redirect('administrator/index.php');

exit();

Editing File: sign_on.php