If your backend trusts what the client sends, it’s already taking a risk.

Clients can be outdated, buggy, or modified. Even when you control the mobile app, you don’t control the environment it runs in. Network tools exist. Apps get reverse-engineered. Assumptions leak.

That’s why validation, authorization, and business rules must live on the server. The client is an interface, not a source of truth.

Many security issues don’t start with attackers doing something clever. They start with engineers assuming, “the client wouldn’t do that.” And then one day, it does.

A safe backend assumes inputs can be wrong, duplicated, or hostile. It doesn’t panic. It just verifies everything and enforces rules consistently.

Security isn’t about distrusting users. It’s about not trusting data blindly.