Vvveb: checkout IDOR allows unauthorized reuse of another user's cart

Vvveb is a powerful and easy to use CMS with page builder to build websites, blogs or ecommerce stores. Prior to 1.0.8.3, the checkout endpoint accepts a user-controlled cart_id and uses it to enter the payment flow without verifying cart ownership. A logged-in attacker can therefore reuse another user's cart data in their own checkout session. This vulnerability is fixed in 1.0.8.3.