Strict Tenant Isolation & Sharding in Multi-Tenant Financial Systems

### Multi-Tenancy Risks in FinTech SaaS

In multi-tenant SaaS architectures, data leaks between competing retail businesses or cross-tenant contamination pose catastrophic compliance and security risks.

A single SQL query lacking a proper `WHERE tenant_id = ?` clause could expose private consumer purchase histories or confidential trade metrics to competitor organizations.

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Rule 11: Every Tenant Check Occurs Server-Side

Voxton enforces Rule 11 across all microservices and API gateways:

> **"Every tenant check occurs server-side."**

Client applications (web frontend, mobile apps) are never trusted to specify or filter tenant scopes. All authentication tokens contain cryptographically signed JWT claims with tenant context verified by NestJS server-side guards.

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Technical Guardrails Implemented

1. **Server-Side Interceptors**: The `TenantInterceptor` validates tenant UUIDs against database schema boundaries on every incoming HTTP request. 2. **Strict Row-Level & Schema Segregation**: Database queries are strictly scoped using PostgreSQL Row Level Security (RLS) or dedicated schema sharding. 3. **Zero Direct Score Edits**: Neither businesses (Rule 3), subscription tiers (Rule 4), nor AI agents (Rule 9) can directly alter numerical trust scores. All score changes are deterministic outputs of the `@voxton/score-engine` package based solely on verified event logs.

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Production Audit Readiness

Every tenant access attempt—whether successful or denied—is written to an immutable server-side log, allowing UK compliance officers and enterprise security teams to verify strict tenant isolation during annual ISO 27001 and Cyber Essentials audits.