Security by Design in Layer Brett Systems

Security is stronger when it is invisible in daily work. Layer Brett bakes security into boundaries and contracts so teams ship safely by default. This post lays out concrete practices you can apply at each layer to protect data, reduce blast radius, and recover quickly when incidents happen.

Threat modeling that matches layers

Run lightweight threat modeling whenever you introduce or change a contract. Ask three questions: what can go wrong at this boundary, how would we notice, and what would limit damage? Keep the model short and tied to the port. For a PaymentPort, focus on replay, tampering, leakage, and repudiation. Document mitigations alongside the interface so they live where developers work.

Secure the interface layer

• Input validation: enforce strict schemas; reject unknown fields to avoid injection.
• Authentication: terminate auth at the edge and pass only the claims the domain needs.
• Rate limiting: apply per-tenant and per-endpoint limits; include backoff headers.
• Idempotency: use keys to prevent replay; bind them to the caller and request hash.
• Transport: enable TLS 1.2+ with strong ciphers; prefer HSTS and secure cookies.

Application layer: orchestrate with guardrails

The application layer sequences use cases, so it is a natural place to enforce cross-cutting controls:

• Authorization checks early, using roles or ABAC policies with clear decision logs.
• Security events: emit structured audit events for sensitive operations.
• Fail closed: on ambiguous states or dependency timeouts, prefer safe failure modes.

Domain layer: pure logic, explicit invariants

Because the domain owns rules, it must encode security-sensitive invariants: balance must never go negative, orders cannot move to shipped without a paid status, and so on. Domain services should be deterministic and free of side effects, making them easier to reason about and verify via tests.

Infrastructure layer: least privilege and segregation

Adapters talk to the world; treat them as the riskiest place. Apply least privilege on every credential and network path. Separate write and read roles. Scope tokens to specific operations. Network segmentation should make it impossible for a compromised adapter to reach unrelated data stores. Prefer short-lived credentials and rotate secrets automatically.

Secrets and configuration hygiene

• Store secrets in a managed vault; never in code or CI variables without encryption.
• Use dynamic credentials with limited TTL; revoke on demand during incidents.
• Separate config from code; sign configuration bundles and verify at startup.

Secure defaults, always

Decision fatigue is the enemy of security. Build secure defaults into shared libraries: HTTPS clients that verify certificates, JSON parsers that reject unknown fields, and SQL helpers that always use parameterized queries. Make the easy path the safe path.

Verifiable policies

Policies should be testable. Write authorization policies as code and include unit tests. Add contract tests that ensure sensitive ports require the right claims. During CI, run static analysis to spot insecure patterns and dependency scans to catch known vulnerabilities. Block merges when critical issues appear.

Observability for security

Security incidents rarely announce themselves. Instrument boundaries so anomalies stand out:

• Audit trails with stable IDs, actor claims, and decision outcomes.
• Metrics for auth failures, rate limit hits, and idempotency replays.
• Traces that include policy version and adapter names to speed triage.

Incident response that respects layers

When something goes wrong, you need switchboards. Put kill switches in adapters to disable specific operations or providers. Provide feature flags at the interface layer to block endpoints or tenants quickly. Make rollbacks boring with versioned contracts and reversible migrations.

Supply chain awareness

Adapters often bring third-party SDKs. Track them like dependencies with SBOMs, pin versions, and monitor advisories. In CI, verify checksums and signatures for artifacts. Avoid transitive surprises by periodically trimming unused dependencies.

Practical checklist

• Define security expectations in each port’s README.
• Add contract tests for auth, rate limits, and error codes.
• Use short-lived, scoped credentials per adapter.
• Log security events with consistent fields and retention.
• Rehearse incident switches in staging monthly.

Closing note

Security by design in Layer Brett is not a separate track of work. It is a series of defaults aligned with your architecture. Keep it close to contracts, make it verifiable, and let automation carry the load. You will ship faster and sleep better.