Program: CWE-22
Project: CVE-2021-44665
Buggy source:
<?php
/**
 * Licensed to The Apereo Foundation under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.

 * The Apereo Foundation licenses this file to you under the Apache License,
 * Version 2.0 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at:
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
require_once(dirname(__FILE__) . "/config.php");
require $xerte_toolkits_site->php_library_path . "user_library.php";
require $xerte_toolkits_site->php_library_path . "template_library.php";
require $xerte_toolkits_site->php_library_path . "template_status.php";

// be slightly paranoid over the path the user is requesting to download.
$unsafe_file_path = $_GET['file'];
if(!preg_match('/^([0-9]+)-([a-z0-9]+)-/', $unsafe_file_path, $matches)) {
    die("path must start with a number, and then a username - e.g. 20-foobar-");
}

$template_id = $matches[1];
$username = $matches[2];

$has_perms = is_user_admin() || has_rights_to_this_template($template_id,$_SESSION['toolkits_logon_id']);

if($has_perms) { 
    if(is_user_an_editor($template_id,$_SESSION['toolkits_logon_id'])){
        if($username == $_SESSION['toolkits_logon_username']) { 
            // they're logged in, and hopefully have access to the media contents.
            $file = dirname(__FILE__) . '/USER-FILES/' . $unsafe_file_path;
            if(!is_file($file)) { 
                die("Fail: file not found on disk"); 
            }
            $filename = addslashes(basename($file));

            header("Cache-Control: public");
            header("Content-Length: " . filesize($file));
            header("Content-Description: File Transfer");
            header("Content-Type: application/force-download"); 
            header("Content-Disposition: attachment; filename=\"$filename\"");
            header("Content-Transfer-Encoding: binary");
            flush();
            readfile($file);
            exit(0);
        }
    }
}

echo "You do not appear to have permission to view this resource.";


Fixed version (ground truth):
<?php
/**
 * Licensed to The Apereo Foundation under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.

 * The Apereo Foundation licenses this file to you under the Apache License,
 * Version 2.0 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at:
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
require_once(dirname(__FILE__) . "/config.php");
require $xerte_toolkits_site->php_library_path . "user_library.php";
require $xerte_toolkits_site->php_library_path . "template_library.php";
require $xerte_toolkits_site->php_library_path . "template_status.php";

// be VERY paranoid over the path the user is requesting to download.
// Even if the file starts with a correct pattern (old implementation) the user could travers path
// like 542-tom-Notingham/../database.php or like 542-tom-Notingham/../../../../etc/passwd
$unsafe_file_path = $_GET['file'];
$full_unsafe_file_path = $xerte_toolkits_site->root_file_path . $xerte_toolkits_site->users_file_area_short . $unsafe_file_path;
// This gets the canonical file name, so in case of 542-tom-Notingham/../../../../etc/passwd -> /etc/passwd
$realpath = realpath($full_unsafe_file_path);
// Check that is start with root_path/USER-FILES
if ($realpath !== false && $realpath === $full_unsafe_file_path) {
    if (!preg_match('/^([0-9]+)-([a-z0-9]+)-/', $unsafe_file_path, $matches)) {
        die("path must start with a number, and then a username - e.g. 20-foobar-");
    }

    $template_id = $matches[1];
    $username = $matches[2];

    $has_perms = is_user_admin() || has_rights_to_this_template($template_id, $_SESSION['toolkits_logon_id']);

    if ($has_perms) {
        if (is_user_an_editor($template_id, $_SESSION['toolkits_logon_id'])) {
            if ($username == $_SESSION['toolkits_logon_username']) {
                // they're logged in, and hopefully have access to the media contents.
                $file = dirname(__FILE__) . '/USER-FILES/' . $unsafe_file_path;
                if (!is_file($file)) {
                    die("Fail: file not found on disk");
                }
                $filename = addslashes(basename($file));

                header("Cache-Control: public");
                header("Content-Length: " . filesize($file));
                header("Content-Description: File Transfer");
                header("Content-Type: application/force-download");
                header("Content-Disposition: attachment; filename=\"$filename\"");
                header("Content-Transfer-Encoding: binary");
                flush();
                readfile($file);
                exit(0);
            }
        }
    }
}
echo "You do not appear to have permission to view this resource.";