// GLOSSARY

Customer Boundary

The authorization boundary separating one customer's data from another's in a multi-tenant application. This is the core trust promise of modern SaaS architecture: hundreds of customers sharing the same infrastructure, database and application code can never see one another's data. But this boundary is logical, not physical; it depends on the code making the right decision on every request. The critical fallacy is believing that authentication establishes this boundary on its own; yet authentication knows who the user is, not whether that user has a right to this very record. The second check, authorization, must be performed separately at every object access; skipped once, an authenticated user can reach into another customer's data. The most expensive form of the BOLA (Broken Object Level Authorization) flaw arises right here: a single missing authority check can collapse the entire customer isolation, among the gravest findings possible in a multi-tenant product. So the customer boundary is tested not with an automated scanner but with manual validation that follows the business logic.