During а pre-lаunch security review оf Nоrthfield Retаil's checkоut platform, the audit team finds a customer-profile page that renders a shopper's saved display name straight into the HTML. A developer on the call argues it needs no special handling because the value came from Northfield's own database and is therefore trusted — until the auditor points out that a display name saved months earlier is already firing a stored XSS payload in every visitor's browser. What principle should have been applied?