The PDF Invoices & Packing Slips for WooCommerce plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 5.6.0 via the `wpo_ips_edi_save_order_customer_peppol_identifiers` AJAX action due to missing capability checks and order ownership validation. This makes it possible for authenticated attackers, with Subscriber-level access and above, to modify Peppol/EDI endpoint identifiers (`peppol_endpoint_id`, `peppol_endpoint_eas`) for any customer by specifying an arbitrary `order_id` parameter on systems using Peppol invoicing. This can affect order routing on the Peppol network and may result in payment disruptions and data leakage.