Authentication

Authentication answers one question: who is making this request? Every answer it produces is an input to authorization, never a substitute for it. A skill that conflates the two produces an app where being logged in is the only permission that exists.

When this fires

A credential is issued, accepted, refreshed or revoked. Adding login, wiring an identity provider, introducing an API key or token, changing session lifetime or cookie flags. It does not fire for "this user should not see that" — that is authorization.

Procedure

1. Find the mechanism that already exists before adding one. Grep for session middleware, an auth library in the dependency manifest, existing cookie names, a provider SDK, Authorization header handling. Most "add auth" tasks are "extend auth". Two parallel auth systems is how a bypass gets built.
2. Name the client type, in writing, before any code. First-party browser app → server session with a cookie. Third-party, native or mobile client → bearer token. Service to service → client-credentials token or mutual TLS. This choice determines everything below; making it implicitly is how a browser app ends up storing a long-lived token in localStorage.
3. Use the framework's or provider's implementation. Do not write password hashing, token signing, or an OAuth flow yourself. If passwords are stored at all, hash with a memory-hard algorithm (argon2id, scrypt, bcrypt) through a maintained library — never a bare SHA, never a homemade salt scheme. When the library's current API matters, look it up rather than recalling it; auth APIs change and a stale call silently weakens the result.
4. Sessions: identifier from a CSPRNG, state held server-side or in a signed cookie you can invalidate. Cookie flags HttpOnly, Secure, SameSite, a scoped Path, and an explicit expiry — all four, not three. Rotate the session identifier on login and on any privilege change. Logout deletes server-side state, not just the cookie.
5. Tokens: validate signature, issuer, audience and expiry on every request, and pin the accepted algorithm on your side — never take the algorithm from the token's own header, and reject none. Verify against the provider's published key set, fetched and cached with rotation in mind. Short access-token lifetime is the containment; a stateless token cannot be recalled once issued.
6. Refresh and revocation are one design, not two. Keep refresh tokens in server-side state so they can be revoked, rotate them on each use, and treat reuse of a rotated token as theft: invalidate the family and force re-login. If you cannot revoke, say so plainly rather than implying you can.
7. OAuth / OIDC: authorization code with PKCE for anything user-facing. Implicit flow and resource-owner password grant are not options. Validate state on the callback, validate nonce in the ID token, and exact-match the redirect URI against an allowlist — prefix matching is an open redirect. The client secret never reaches a browser or a shipped binary; code is exchanged for tokens server-side.
8. Resolve identity at one choke point. Middleware turns a request into a principal or into anonymous, once. Handlers read that principal; they never parse the raw credential themselves. Scattered credential parsing guarantees one handler parses it differently.
9. Fail closed and fail uniformly. Missing or bad credential → 401, generic message. Do not reveal whether an account exists, and keep the response shape and timing identical for unknown user and wrong password. Rate-limit and back off on credential endpoints, including password reset and token exchange.
10. Secrets come from the environment or a secret manager. Never a committed file, never a default baked into code. Before rotating a signing key, changing a cookie name or shortening session lifetime on a running system, stop and ask — each of those logs every user out.
11. Prove it by executing it. An auth change is verified by requests actually sent: valid credential, absent credential, malformed credential, expired credential, and the refresh path. Reading the diff is a review. Hand the proof to api-contract-verification.

Checklist

• Existing mechanism found and extended, or its absence confirmed
• Client type named, and the session/token choice follows from it
• No hand-written crypto; library and its current API confirmed
• Cookie flags set, or token validation pins issuer, audience, expiry and algorithm
• Session identifier rotates on login and privilege change
• Refresh tokens revocable and rotated, or the absence of revocation stated
• OAuth: PKCE, state, nonce, exact redirect-URI match, secret server-side only
• One resolution point; handlers read a principal, not a credential
• Unauthenticated responses uniform, generic, rate-limited
• Secrets out of the repository; anything that logs users out was asked about first
• Valid, missing, malformed, expired and refresh paths executed, not reasoned about

Failure handling

• Login works but the session does not persist — read the actual Set-Cookie on the wire before touching code. Domain, Path, Secure over plain HTTP, and SameSite on a cross-site callback account for most of these, and none of them are visible in the source.
• Token validates in one service and not another — compare issuer, audience and clock skew before suspecting the key. Shared secrets that "work locally" usually mean validation is being skipped somewhere.
• The provider's flow does not behave as documented — capture the exact callback parameters and error code and report them. Do not loosen redirect-URI matching, disable state checks, or skip signature verification to get past it. That is not a workaround, it is the vulnerability.
• No way to test the provider without production credentials — say the flow is unverified and name which step is unproven. Do not report an OAuth integration as working because the code compiles.

Evidence to report

Which mechanism is in use and why that client type chose it; the cookie flags or the exact token claims validated; where identity is resolved, as a file and function; the requests actually executed and their statuses, including the failure and refresh paths; anything that could not be executed, named. "Auth added" with none of that is a claim, not evidence.