Week 8: Integrations and Security-Sensitive Flows

Learning Objectives

• Understand Stripe checkout and payment intent flow boundaries.
• Apply correct authorization checks on sensitive endpoints.
• Implement webhook signature verification and trust boundaries.
• Design idempotent handlers for retry-safe behavior.
• Log security events without leaking secrets.

Detailed Topics

1. Stripe Integration Patterns

Cover client-to-server payment lifecycle, key separation, and safe handling of payment status transitions. Emphasize server-confirmed source of truth over client assumptions.

2. Authorization-Related Flows

Teach that authentication answers "who are you" while authorization answers "are you allowed." Every account, order, or payment endpoint should validate ownership and role constraints.

3. Webhook Security

Webhooks are inbound privileged messages. Validate provider signatures, reject unsigned or stale payloads, and isolate parsing from business side effects.

4. Idempotency

External systems retry. Handlers must be safe under repeated delivery. Store idempotency keys or event IDs to ensure repeated calls do not duplicate side effects.

5. Signature and Authorization Awareness

Distinguish authenticity from permissions. A valid webhook signature proves source authenticity, not business authorization. Internal endpoints still need explicit permission checks.

1. Read raw body exactly as required by provider SDK.
2. Verify signature with provider secret.
3. Reject invalid signatures with a safe error response.
4. Check event age tolerance to reduce replay risk.
5. Enforce idempotency using event ID or request key.
6. Route only supported event types.
7. Log event metadata (not secrets or full payloads).
8. Return deterministic status for retry behavior.

Pair Session Plan (90 Minutes)

1. Review one auth/payment/webhook endpoint.
2. Add explicit authorization and validation checks.
3. Add signature verification or improve existing checks.
4. Implement idempotency guard and verify repeated requests.

Independent Ticket

Harden one integration-sensitive API path and submit a security checklist with threat notes and verification steps.

Mentor Checkpoint

• Student can explain trust boundaries clearly.
• Student differentiates authentication and authorization.
• Student demonstrates replay-safe handling.
• Student avoids logging secrets or sensitive PII.