Spree is an open source e-commerce solution built with Ruby on Rails. A critical IDOR vulnerability exists in Spree Commerce's guest checkout flow that allows any guest user to bind arbitrary guest addresses to their order by manipulating address ID parameters. This enables unauthorized access to other guests' personally identifiable information (PII) including names, addresses and phone numbers. The vulnerability bypasses existing ownership validation checks and affects all guest checkout transactions. This vulnerability is fixed in 4.10.3, 5.0.8, 5.1.10, 5.2.7, and 5.3.2.