Most systems assume: “Requests coming from our app are safe.” That assumption breaks quickly. Mobile apps handle validation, flows, and restrictions — but the client environment is controllable. Requests can be modified, replayed, or triggered outside the UI. The real issue isn’t missing validation — it’s misplaced trust. What goes wrong Client-side validation gets bypassed Flows are executed out of sequence Parameters are tampered Requests are replayed Example ❌ Vulnerable: if (req.isKycVerified) { generateLink(); } Enter fullscreen mode Exit fullscreen mode Client controls this flag. ✅ Correct: User user = repo.find(req.userId); if (user.getKycStatus() == VERIFIED) { generateLink(); } Enter fullscreen mode Exit fullscreen mode What to fix Backend must be the source of truth Validate all critical data server-side Enforce flows on backend Add idempotency for sensitive operations Key takeaway UI constraints are not security controls.…