CVE-2026-109217: 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 'invoicenumber (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.