Hands-On Lab and Phase Exit Implement, verify, explain, then prepare for authorization
Course principle: a user ID becomes trusted only after the access token has been cryptographically verified by the backend.
Level 1 - Follow Install the backend Supabase client, add API environment variables, create SupabaseAuthService, create the guard, add @CurrentUser(), and protect /auth/me.
Level 2 - Modify Protect TasksController at controller level. Keep /health public. Verify the no-token and valid-token paths.
Level 3 - Implement Build an authenticated GET /auth/me response that returns only safe verified identity fields. Add a frontend page that calls it from Next.js using the current access token.
Level 4 - Engineer Design the production authentication policy for a payroll API containing public health checks, employee endpoints, HR endpoints, and internal automation endpoints. Decide whether auth should
be global/default-deny and identify which endpoints truly need different credential types.
Phase exit oral questions • Why is a userId query parameter not identity?
• What exactly turns claims.sub into trusted identity?
• Why is decode-only JWT handling unsafe?
• Why does a 401 differ from a 403?
• Why is CORS not authentication?
• Why do we keep authorization out of AuthGuard?
• When would direct JWKS verification be appropriate?
Exit checklist • Valid Supabase token reaches NestJS as a Bearer token.
• Missing/malformed/expired tokens return 401.
• Verified sub is attached as request user.id.
• Protected Tasks routes reject anonymous callers.
• Health route remains public.
• No access token or secret key is committed/logged.
• You can explain the difference between authentication, authorization and business rules.
Phase 18 is complete when NestJS can establish a trusted caller identity independently of the browser and independently of Next.js.
Source note: verified against current Supabase Auth/getClaims/signing-key guidance and NestJS authentication/authorization docs. Full URLs and version notes are included in SOURCE_NOTES.txt.
Authentication answers who the request represents. Treating decoded or client-supplied identity as trusted is a security boundary failure.
Restate the goal of “Hands On Lab and Phase Exit” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: credentials/session → token → verification → trusted identity → protected operation. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Authentication guard + verification service + current-user helper + protected /auth/me and task routes.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
const token = readBearerToken(request);
const claims = await verifier.verify(token);
request.user = {
id: claims.sub,
email: claims.email,
};Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Inspect whether a token exists, whether it is expired, which issuer/audience signed it, and what the API verifier expects.
Trust only verified server claims; never repair identity by accepting a client-sent user id.
Without copying the final source, create a second small scenario that applies “Hands On Lab and Phase Exit” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Attach identity once and expose it through a typed decorator/helper rather than reparsing tokens in services.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Returning 403 when no valid authentication exists
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.