Sessions, Cookies & Browser Storage Understand authenticated continuity and the security consequences of storage choices
Phase 16 - Authentication Fundamentals Volume 05 - Authentication & Security
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Phase 16 builds the security mental model before any Supabase Auth integration code is introduced.
HTTP is stateless; authenticated applications need continuity After login, the server needs a way to associate later HTTP requests with the authenticated subject. A session provides that continuity. The session can be represented by an opaque identifier stored server-side, by tokens, or by a provider-specific combination.
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The browser holds an identifier; the authoritative session state remains server-side.
Cookie security attributes Attribute What it protects / controls
Path / Domain Restricts where the cookie is sent; use the narrowest practical scope
Set-Cookie: __Host-taskflow_session=<opaque-id>; Path=/; Secure; HttpOnly; SameSite=Lax
Phase 17 will use Supabase Auth and its current SSR/browser integration guidance. Do not copy this cookie as a custom authentication system.
Browser storage is a security decision localStorage and sessionStorage are readable by JavaScript executing in the origin. OWASP currently advises not storing authentication tokens, session identifiers, JWTs, or refresh tokens there when a safer HttpOnly cookie or backend-for-frontend/session pattern is available. XSS can read JavaScript-accessible storage.
Storage Automatically sent? JavaScript readable? Important risk
Non-HttpOnly cookie Yes Yes XSS can read it; CSRF also relevant
localStorage No Yes XSS can exfiltrate credential/token
sessionStorage No Yes XSS can exfiltrate credential/token
Server memory/session No browser access No Requires server-side session management store
Cookie security is not one checkbox HttpOnly helps keep JavaScript from reading a cookie, but it does not stop the browser from sending that cookie automatically. That is why cookie authentication still requires intentional SameSite and CSRF design. Security properties trade off against each other; storage choice must match application architecture.
Authentication answers who the request represents. Treating decoded or client-supplied identity as trusted is a security boundary failure.
Restate the goal of “Sessions Cookies and Browser Storage” 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 threat/flow diagram explaining what the browser stores, what servers verify, and what happens when credentials expire.. 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 “Sessions Cookies and Browser Storage” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Explain secure/httpOnly/sameSite cookie attributes and when browser JS should not read a token.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Confusing hidden UI with authorization
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.