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.

Affected: RubyGems: spree_api

## Summary

The Store API v3 endpoint `PATCH /api/v3/store/carts/:id/associate` binds a guest cart to the authenticated caller without verifying possession of that cart. It locates the cart by prefixed ID only — `current_store.carts.where(user: [nil, current_user]).find_by_prefix_id!(params[:id])` — and omits the `authorize!(:update, @cart, cart_token)` check that every other action in the controller performs via `CartResolvable`. Because prefixed IDs are a reversible Sqids encoding of the auto-increment primary key (obfuscation, not a token), an authenticated customer can name arbitrary guest cart IDs, take them over, and read the checkout addresses stored on them. This is broken access control / IDOR, reachable by any low-privilege registered user.

## Severity

Requires an authenticated store account and depends on target guest carts already carrying an address and not yet being associated, on a store not running in `login_required` mode. Confidentiality impact is the driver (guest checkout PII); integrity impact is limited and recoverable (cart reassignment + email overwrite on an in-progress cart). Not Critical: the action is gated behind authentication (`PR:L`, not `PR:N`) and constrained by cart state, so it is not anonymously exploitable.

## Details

**Root cause:** `associate` skips the cart-possession check its sibling actions enforce and trusts a guessable identifier as the sole locator.

**Entry point.** `Spree::Api::V3::Store::CartsController#associate` (`carts…