Tutor LMS < 3.9.8 - Unauthenticated Arbitrary Billing Profile Overwrite via 'order_id'

Description

The plugin is vulnerable to an Insecure Direct Object Reference due to missing authentication and authorization checks in the `pay_incomplete_order()` function. The function accepts an attacker-controlled `order_id` parameter and uses it to look up order data, then writes billing fields to the order owner's profile (`$order_data->user_id`) without verifying the requester's identity or ownership. Because the Tutor nonce (`_tutor_nonce`) is exposed on public frontend pages, this makes it possible for unauthenticated attackers to overwrite the billing profile (name, email, phone, address) of any user who has an incomplete manual order, by sending a crafted POST request with a guessed or enumerated `order_id`.