The Upsell Funnel Builder for WooCommerce plugin for WordPress is vulnerable to order manipulation in all versions up to, and including, 3.0.0. This is due to the plugin allowing the additional product ID and discount field to be manipulated prior to processing via the add_offer_in_cart function. This makes it possible for unauthenticated attackers to arbitrarily update the product associated with any order bump, and arbitrarily update the discount applied to any order bump item, when adding it to the cart.
The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.
If a web product does not properly protect assumed-immutable values from modification in hidden form fields, parameters, cookies, or URLs, this can lead to modification of critical data. Web applications often mistakenly make the assumption that data passed to the client in hidden fields or cookies is not susceptible to tampering. Improper validation of data that are user-controllable can lead to the application processing incorrect, and often malicious, input. For example, custom cookies commonly store session data or persistent data across sessions. This kind of session data is normally involved in security related decisions on the server side, such as user authentication and access control. Thus, the cookies might contain sensitive data such as user credentials and privileges. This is a dangerous practice, as it can often lead to improper reliance on the value of the client-provided cookie by the server side application.