From Session to Trusted API Identity Why a Next.js login is not enough for NestJS
Course principle: a user ID becomes trusted only after the access token has been cryptographically verified by the backend.
Two separate trust boundaries TaskFlow has two server processes. Next.js can read its cookie-backed Supabase session, while NestJS receives an ordinary HTTP request. NestJS must establish trust independently.
• cookie session • receives headers
• refresh lifecycle • no cookie knowledge assumed
• Supabase SSR client • must verify bearer JWT
• can obtain access token • creates request identity
Wrong architecture GET /tasks?userId=5c53...
// Browser claims who the user is. // The API trusts an unverified identifier.
A UUID is not proof of identity. Anyone can send someone else's UUID.
Correct architecture Identity establishment
Authentication vs authorization Question Layer Typical result
Who is calling? Auth guard 401 when not authenticated
May this user access this resource? Authorization/business layer 403 when authenticated but forbidden
Is the requested workflow valid?Service/business rules 400/409 depending on rule
After verification, claims.sub may be used as the authenticated Supabase user identifier. Before verification, decoded JWT contents are attacker-controlled input.
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 “From Session to Trusted API Identity” 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 “From Session to Trusted API Identity” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Add Authorization: Bearer token handling in the API client without putting tokens in URLs/logs.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Trusting userId from request body/query
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.