Quick Playground <= 1.3.4 - Authenticated (Administrator+) Arbitrary File Read via 'filename' Parameter

The Quick Playground plugin for WordPress is vulnerable to Path Traversal in all versions up to, and including, 1.3.4. This is due to the `qckply_data()` function passing the user-supplied `filename` POST parameter directly to `file_get_contents()` without any validation, sanitization, or path restriction. This makes it possible for authenticated attackers, with Administrator-level access and above, to read arbitrary files on the server, such as `wp-config.php` or `/etc/passwd`, which can contain sensitive information. Note: This vulnerability is only exploitable when the site has been synced with WordPress Playground (the `is_qckply_clone` option is set) or when running on `playground.wordpress.net`.