server/apps/files_sharing/ajax/publicpreview.php

<?php
/**
 * @author Björn Schießle <schiessle@owncloud.com>
 * @author Georg Ehrke <georg@owncloud.com>
 * @author Lukas Reschke <lukas@owncloud.com>
 * @author Morris Jobke <hey@morrisjobke.de>
 * @author Thomas Müller <thomas.mueller@tmit.eu>
 *
 * @copyright Copyright (c) 2015, ownCloud, Inc.
 * @license AGPL-3.0
 *
 * This code is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * This program 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License, version 3,
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 *
 */

OCP\JSON::checkAppEnabled('files_sharing');

\OC_User::setIncognitoMode(true);

$file = array_key_exists('file', $_GET) ? (string) $_GET['file'] : '';
$maxX = array_key_exists('x', $_GET) ? (int) $_GET['x'] : '36';
$maxY = array_key_exists('y', $_GET) ? (int) $_GET['y'] : '36';
$scalingUp = array_key_exists('scalingup', $_GET) ? (bool) $_GET['scalingup'] : true;
$token = array_key_exists('t', $_GET) ? (string) $_GET['t'] : '';
$keepAspect = array_key_exists('a', $_GET) ? true : false;

if($token === ''){
	\OC_Response::setStatus(\OC_Response::STATUS_BAD_REQUEST);
	\OCP\Util::writeLog('core-preview', 'No token parameter was passed', \OCP\Util::DEBUG);
	exit;
}

$linkedItem = \OCP\Share::getShareByToken($token);
if($linkedItem === false || ($linkedItem['item_type'] !== 'file' && $linkedItem['item_type'] !== 'folder')) {
	\OC_Response::setStatus(\OC_Response::STATUS_NOT_FOUND);
	\OCP\Util::writeLog('core-preview', 'Passed token parameter is not valid', \OCP\Util::DEBUG);
	exit;
}

if(!isset($linkedItem['uid_owner']) || !isset($linkedItem['file_source'])) {
	\OC_Response::setStatus(\OC_Response::STATUS_INTERNAL_SERVER_ERROR);
	\OCP\Util::writeLog('core-preview', 'Passed token seems to be valid, but it does not contain all necessary information . ("' . $token . '")', \OCP\Util::WARN);
	exit;
}

$rootLinkItem = OCP\Share::resolveReShare($linkedItem);
$userId = $rootLinkItem['uid_owner'];

OCP\JSON::checkUserExists($rootLinkItem['uid_owner']);
\OC_Util::setupFS($userId);
\OC\Files\Filesystem::initMountPoints($userId);
$view = new \OC\Files\View('/' . $userId . '/files');

$pathId = $linkedItem['file_source'];
$path = $view->getPath($pathId);

if($path === null) {
	\OC_Response::setStatus(\OC_Response::STATUS_NOT_FOUND);
	\OCP\Util::writeLog('core-preview', 'Could not resolve file for shared item', \OCP\Util::WARN);
	exit;
}

$pathInfo = $view->getFileInfo($path);
$sharedFile = null;

if($linkedItem['item_type'] === 'folder') {
	$isValid = \OC\Files\Filesystem::isValidPath($file);
	if(!$isValid) {
		\OC_Response::setStatus(\OC_Response::STATUS_BAD_REQUEST);
		\OCP\Util::writeLog('core-preview', 'Passed filename is not valid, might be malicious (file:"' . $file . '";ip:"' . \OC::$server->getRequest()->getRemoteAddress() . '")', \OCP\Util::WARN);
		exit;
	}
	$sharedFile = \OC\Files\Filesystem::normalizePath($file);
}

if($linkedItem['item_type'] === 'file') {
	$parent = $pathInfo['parent'];
	$path = $view->getPath($parent);
	$sharedFile = $pathInfo['name'];
}

$path = \OC\Files\Filesystem::normalizePath($path, false);
if(substr($path, 0, 1) === '/') {
	$path = substr($path, 1);
}

if($maxX === 0 || $maxY === 0) {
	\OC_Response::setStatus(\OC_Response::STATUS_BAD_REQUEST);
	\OCP\Util::writeLog('core-preview', 'x and/or y set to 0', \OCP\Util::DEBUG);
	exit;
}

$root = 'files/' . $path;

try{
	$preview = new \OC\Preview($userId, $root);
	$preview->setFile($sharedFile);
	$preview->setMaxX($maxX);
	$preview->setMaxY($maxY);
	$preview->setScalingUp($scalingUp);
	$preview->setKeepAspect($keepAspect);

	$preview->showPreview();
} catch (\Exception $e) {
	\OC_Response::setStatus(\OC_Response::STATUS_INTERNAL_SERVER_ERROR);
	\OCP\Util::writeLog('core', $e->getmessage(), \OCP\Util::DEBUG);
}