Program: CWE-89
Project: CVE-2022-21720
Buggy source:
<?php
/**
 * ---------------------------------------------------------------------
 * GLPI - Gestionnaire Libre de Parc Informatique
 * Copyright (C) 2015-2022 Teclib' and contributors.
 *
 * http://glpi-project.org
 *
 * based on GLPI - Gestionnaire Libre de Parc Informatique
 * Copyright (C) 2003-2014 by the INDEPNET Development Team.
 *
 * ---------------------------------------------------------------------
 *
 * LICENSE
 *
 * This file is part of GLPI.
 *
 * GLPI is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * GLPI is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GLPI. If not, see <http://www.gnu.org/licenses/>.
 * ---------------------------------------------------------------------
 */

include ('../inc/includes.php');

$dropdown = new Entity();

// Root entity : no delete
if (isset($_GET['id']) && ($_GET['id'] == 0)) {
   $options = ['canedit' => true,
                    'candel'  => false];
}

if (array_key_exists('custom_css_code', $_POST)) {
   $_POST['custom_css_code'] = $_UPOST['custom_css_code'];
}

include (GLPI_ROOT . "/front/dropdown.common.form.php");


Fixed version (ground truth):
<?php
/**
 * ---------------------------------------------------------------------
 * GLPI - Gestionnaire Libre de Parc Informatique
 * Copyright (C) 2015-2022 Teclib' and contributors.
 *
 * http://glpi-project.org
 *
 * based on GLPI - Gestionnaire Libre de Parc Informatique
 * Copyright (C) 2003-2014 by the INDEPNET Development Team.
 *
 * ---------------------------------------------------------------------
 *
 * LICENSE
 *
 * This file is part of GLPI.
 *
 * GLPI is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * GLPI is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GLPI. If not, see <http://www.gnu.org/licenses/>.
 * ---------------------------------------------------------------------
 */

include ('../inc/includes.php');

$dropdown = new Entity();

// Root entity : no delete
if (isset($_GET['id']) && ($_GET['id'] == 0)) {
   $options = ['canedit' => true,
                    'candel'  => false];
}

if (array_key_exists('custom_css_code', $_POST)) {
   // Prevent sanitize process to alter '<', '>' and '&' chars.
   $_POST['custom_css_code'] = $DB->escape($_UPOST['custom_css_code']);
}

include (GLPI_ROOT . "/front/dropdown.common.form.php");