The PeproDev Ultimate Invoice plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 2.0.9 via the invoicing viewer due to missing validation on a user controlled key. This makes it possible for unauthenticated attackers to view invoices for completed orders which can contain PII of users.
The PeproDev Ultimate Invoice WordPress plugin through 2.2.5 has a bulk download invoices action that generates ZIP archives containing exported invoice PDFs. The ZIP files are named predictably making it possible to brute force and retreive PII.