The Klamra Paycal for Aspaclaria plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 1.1.4 via the 'invoice_id' parameter due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with subscriber-level access and above, to download arbitrary customer invoices by enumerating sequential post IDs, exposing sensitive billing PII including full name, email address, phone number, order total, line items, and customer notes belonging to other customers.