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For a tenant-scoped request, derive the tenant from server-verified identity and authorization—not from a tenant ID the caller supplies. A value such as {"tenant_id":"acme"} can identify the tenant the caller wants, but it does not prove the caller may act for Acme. Verify the caller’s current membership or explicitly scoped service authorization, then enforce that trusted tenant context throughout the request.
Why a request-supplied tenant ID is not proof of access
Request bodies, headers, and query parameters are controlled by the caller. If a handler reads tenant_id from one of them and uses it to scope a database query, it has selected a tenant without establishing that the principal is allowed to use it. Treat such a value only as a selector: compare it with the tenant the authenticated principal is authorized to use, and reject a mismatch. OWASP’s Multi-Tenant Application Security Cheat Sheet describes this distinction and recommends binding tenant context to verified identity and active membership.
Authentication answers who the caller is; authorization answers whether that caller may perform this action on this resource. A valid login or token does not automatically grant access to every tenant or every object. OWASP’s Authorization Cheat Sheet advises checking authorization on every request and for the resource being accessed.
Establish trusted tenant context in the request flow
- Authenticate first. Use the server’s authentication layer to establish the principal and validate credentials.
- Choose the tenant from trusted information. Use verified claims only to the extent their issuer guarantees they are suitable for this decision; otherwise resolve the principal’s tenant selection through a current membership check.
- Authorize the relationship. Confirm that the principal currently belongs to the tenant, or that a service identity has explicit authorization for it and the requested operation.
- Bind context to the request. Store the authorized tenant in request-local server context, rather than repeatedly trusting caller-controlled input.
- Enforce it at each access point. Tenant-scoped handlers and data access must require that context and verify resource ownership. If the caller also supplied a tenant selector, ensure it matches the authorized tenant.
Return behavior for missing context or denied membership should follow the API’s contract. OWASP’s example denies a principal without membership; the exact status code and response format are application-specific.
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Authorize tenant-owned resources, not just the tenant label
Checking membership in a tenant is not always enough. For an operation on a tenant-owned invoice, document, or user, the authorization decision must also establish that the specific object belongs to that tenant and that the principal may perform the requested action. Include tenant ownership in the lookup or authorization policy wherever ownership is tenant-specific. Random or opaque object IDs can make guessing harder, but they do not replace authorization.
Preserve or re-establish context across system boundaries
Service-to-service requests
A downstream service should not accept an internal trusted header merely because the caller supplied it. Validate the trusted issuer, integrity, audience, expiry, and whether the tenant context applies to the actual request. A valid signature proves that a value was signed; it does not by itself authorize a different tenant, resource, or action. OWASP’s Authorization Patterns Cheat Sheet covers validation of propagated authorization context.
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- API Security in Action
- Manning Publications
- ABIS BOOK
Tenant-sensitive caches
Derive the tenant identity used for protected cache access from trusted authenticated context. Include tenant identity and other authorization-relevant dimensions in cache keys when they affect the result, and authorize before returning protected cached data. Separate keys reduce accidental cross-tenant reuse, but do not replace an authorization check. See OWASP’s Web Cache Security Cheat Sheet.
Queued and asynchronous work
Carry tenant context from an authenticated producer through a trusted broker route, authenticated metadata, or an integrity-protected payload. When consuming the job, authenticate the producer or broker path, re-establish the trusted context, and authorize both the operation and its target resource. If execution is delayed, recheck membership or permission when the original authorization decision could have become stale.
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Tenant-aware query scoping and database row-level security (RLS) can add enforcement beyond application checks. They are not substitutes for establishing trusted context and authorizing the caller. In PostgreSQL designs that set tenant context through a session setting, OWASP recommends transaction-local context for shared-table request paths: a pooled connection can otherwise retain session state and expose one request’s tenant context to another. Ensure ordinary request roles cannot bypass RLS, and test isolation using the same database role and connection path used in production. OWASP covers these controls in its multi-tenant guidance.
Schema separation or separate infrastructure are also possible isolation choices, but none is universally right. Choose among shared-table controls, schema separation, and separate infrastructure based on the threat model, enforcement coverage, service commitments, and operational cost. Verify that every access path—not only the primary web handler—applies the intended boundary.
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Common shortcuts that do not establish authorization
- Trusting a body, header, or query-string tenant ID: it is caller input, not proof of membership.
- Trusting a signed tenant value alone: signature validation does not establish that the tenant, object, or action is authorized for this request.
- Relying on opaque IDs: harder-to-guess identifiers do not prevent access-control failures.
- Assuming an internal network or shared queue is a boundary: each receiving service or worker still needs authenticated, validated context and authorization.
- Relying only on an ORM filter: isolation must cover every data-access path and be verified under production-like roles and connection behavior.
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