A multi-tenant document vault enforced a real server-side tenant-ownership check on its permission-grant endpoint - and it was still bypassable. The authorization layer read user_group_id as a single value; the ACL writer split the same string on commas and processed every element. Sending two IDs joined by a comma, victim first, granted an external account access across the tenant boundary and leaked their name, email and internal tenant ID on read-back. A walkthrough of the parser differential behind it, a reusable test matrix, and the fixes that actually close the class.