Access Tokens, Refresh Tokens & Bearer Risk Understand short-lived API credentials, refresh lifecycles, and replay risk
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.
Access tokens and refresh tokens have different jobs An access token is presented to a protected resource such as the TaskFlow API. A refresh token is presented to the authorization/authentication system to obtain a new access token. Refresh tokens usually live longer and therefore deserve stronger protection.
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The refresh token should not be sent to every protected API request.
Access token Resource/API server Shorter Stolen token can call API until expiry/revocation constraints
Refresh token Auth/token endpoint Longer Stolen token can mint new access tokens
Bearer means possession is enough RFC 6750 defines bearer-token usage. A bearer token works for whoever possesses it unless additional sender-constraining mechanisms are used. Therefore an access token must be handled like a credential, not like a harmless identifier.
GET /tasks HTTP/1.1 Host: api.example.com Authorization: Bearer <access-token>
URLs leak into browser history, logs, analytics, referrer data, screenshots, and monitoring systems. Use the Authorization header for protected API requests.
Short lifetime limits damage If an access token leaks, a shorter expiry reduces the attack window. OAuth 2.0 Security Best Current Practice also recommends restricting token privileges and audience as much as practical.
Modern OAuth security guidance requires public clients that receive refresh tokens to use sender-constrained refresh tokens or refresh-token rotation. With rotation, each refresh produces a new refresh token and invalidates the prior one, enabling replay detection.
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The exact behavior belongs to the identity provider; application code should not invent its own rotation scheme casually.
Authentication answers who the request represents. Treating decoded or client-supplied identity as trusted is a security boundary failure.
Restate the goal of “Access Refresh Tokens and Bearer Risk” 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 “Access Refresh Tokens and Bearer Risk” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Separate authentication (who) from authorization (may do what) with concrete TaskFlow examples.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Treating base64 JWT content as proof of authenticity
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.