Spree is an open source e-commerce solution built with Ruby on Rails. From 5.4.0 until 5.4.4 and 5.5.4, PATCH /api/v3/store/carts/:id/associate in Spree::Api::V3::Store::CartsController#associate uses find_cart_for_association to locate a cart by prefixed_id but does not require a cart token or otherwise verify possession of the selected guest cart. An authenticated customer can derive reversible prefixed cart IDs, associate an eligible guest cart with the attacker's account, and receive billing and shipping address data from the cart. Exploitation requires a guest cart with address data on a store that does not require login for checkout, and reassignment can also disrupt the guest's in-progress cart. This issue is fixed in versions 5.4.4 and 5.5.4.