FOSSBilling is a billing and client management system that automates invoicing, payments, and communication for online service businesses. Versions 0.6.21 through 0.7.2 are vulnerable to IDOR through the support ticket creation workflow. By manipulating rel_id when rel_type=order, an authenticated client can create a support ticket that references another client's order they do not own. The ticketCreateForClient() method accepted rel_id without verifying order ownership for non-upgrade tasks, allowing clients to link a new ticket to another client's order by crafting the request. No cron task automatically processes cancel/upgrade requests from ticket relations; staff action is required. This affects integrity and confidentiality: staff could be misled into acting on the wrong order (e.g., cancellation or upgrade requests). While there is no client-to-client order data exposure, order IDs may appear in ticket context. This issue has been fixed in version 0.8.0.