Online Invoicing System (OIS) is open source software which is a lean invoicing system for small businesses, consultants and freelancers created using AppGini. In OIS version 4.0 there is a stored XSS which can enables an attacker takeover of the admin account through a payload that extracts a csrf token and sends a request to change password. It has been found that Item description is reflected without sanitization in app/items_view.php which enables the malicious scenario.