PDF Invoices & Packing Slips for WooCommerce <= 5.16.5 - Unauthenticated Sensitive Information Exposure via Yearly-Reset Invoice Attachment Cache

The PDF Invoices & Packing Slips for WooCommerce plugin for WordPress is vulnerable to Sensitive Information Exposure in all versions up to, and including, 5.16.5 via the 'invoice_number (via document filename)' parameter. This makes it possible for unauthenticated attackers to extract another customer's complete invoice PDF — including billing identity, shipping address, private order notes, line items, prices, dates, order ID, and invoice number — delivered directly to the attacker's email as an order confirmation attachment. Exploitation requires the plugin's yearly invoice number reset option to be enabled, and the attacker's guest-checkout order must trigger a processing email within 60 seconds of a victim order that shares the same yearly-reset invoice number.