The original title is about building secure B2B client portals on Google Sheets. Let me rewrite this as a punchy mobile feed headline.

Short summary

Shared Google Sheets with client-side filtering expose critical data isolation vulnerabilities when teams need to provide clients secure, role-based access. This guide details a three-step server-side architecture—bespoke authentication, cloud verification, and isolated data extraction—that prevents cross-client data exposure while maintaining zero hosting costs. Deploy using Google Apps Script with administrative permissions, relational Client ID schema design, and eliminate expensive SaaS portal platforms entirely.

• •Client-side filtering in shared spreadsheets creates exploitable data isolation vulnerabilities
• •Server-side architecture with three-step authentication handshake ensures only matching data reaches each client
• •Relational schema with Client ID keys enables zero-cost B2B portals via Google Apps Script admin execution

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