yes

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magentocommerce.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magentocommerce.com for more information.
 *
 * @category   Mage
 * @package    Mage
 * @copyright  Copyright (c) 2008 Irubin Consulting Inc. DBA Varien (http://www.varien.com)
 * @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

error_reporting(E_ALL | E_STRICT);
define('MAGENTO_ROOT', getcwd());
$mageFilename = MAGENTO_ROOT . '/app/Mage.php';
require_once $mageFilename;
Mage::setIsDeveloperMode(true);//just so you see the errors if any
Mage::app();//instantiate the application






$categoryId = 283;
$category = Mage::getModel('catalog/category')->load($categoryId);
$ids = Mage::getModel('catalog/product')->getCollection()->addCategoryFilter($category)->getAllIds();
$attributeCode = 'brand';
$attributeValue = '5';
Mage::getSingleton('catalog/product_action')->updateAttributes(
    $ids,
    array($attributeCode => $attributeValue),
    0
);

?>