Auth Hardening

Authentication is the front door of an application. Most real breaches don't come from exotic exploits — they come from ordinary mistakes in how login, sessions, and logout are built. This skill's job is to catch those mistakes and replace them with proven patterns, then prove the fix works.

The guiding rule throughout: the server is the only place trust lives. Any check on the client is user experience, not security. If a control isn't enforced server-side, treat it as absent.

When to reach for this skill

Use it the moment a task touches any of: a login or logout endpoint, session cookies or JWTs, password storage or reset, "remember me" / refresh tokens, account lockout, or OTP/MFA. Reviewing existing auth code counts too — you don't need the user to say "harden," just to be working in this area.

Workflow

Work through these five areas in order. Each has a "get it right" standard and a matching failure to hunt for. Don't skip to code before understanding which stack and session model the user has — read what exists first, then apply the standards.

1. Password storage

The standard: hash with a slow, salted, memory-hard algorithm — argon2id (preferred) or bcrypt. Never MD5, SHA-1, SHA-256, or HMAC-SHA256 for passwords. Fast hashes exist to be fast; that is exactly wrong for passwords, because it lets an attacker who steals the database try billions of guesses.

Note the distinction that trips people up: HMAC-SHA256 is a fine choice for signing tokens (short, high-entropy, machine-generated) but a poor one for passwords (human-chosen, low-entropy, needs deliberate slowness). Seeing saltHex:hashHex in an admin_users table is a signal to check whether a fast hash is guarding a human password — if so, migrate to argon2id.

Never roll a custom scheme. Use the platform's vetted library.

2. Session model — the logout problem

First decide the model, because logout behaves differently in each:

Stateful sessions (server stores a session record, client holds an opaque ID in an httpOnly cookie). Logout is easy and correct here: delete the server record and the session is genuinely dead everywhere.

Stateless JWTs (client holds a signed token; server verifies the signature without storing anything). This is where most logout bugs live. A JWT stays valid until it expires no matter what the client does — clearing it from the browser does not invalidate it. A stolen token still works. So real logout requires one of:

• short access-token lifetimes (5–15 min) plus a refresh token you can revoke, or
• a server-side revocation list / token blocklist checked on each request, or
• a per-user token version that you bump on logout so old tokens fail.

The failure to hunt: "logout" that only does localStorage.removeItem('token') with no server-side invalidation. That is not logout; it is hiding the key under the mat.

See references/session-patterns.md for concrete implementations of each model, including refresh-token rotation.

3. Cookie and token handling

If using cookies: set HttpOnly (blocks JS/XSS theft), Secure (HTTPS only), and SameSite=Lax or Strict (blunts CSRF). A token in localStorage is readable by any XSS on the page — prefer httpOnly cookies for session material, and if you must use localStorage, understand you've accepted that risk.

For JWTs: verify the signature with a strong secret or asymmetric key, pin the algorithm server-side (reject alg: none and algorithm-confusion), and validate exp, iss, and aud. Keep access tokens short-lived; put longevity in revocable refresh tokens, and rotate refresh tokens on use so a stolen one is single-use.

4. Brute-force and enumeration resistance

Login and reset endpoints are guessing targets. Defenses:

• Rate limiting per account and per IP, with escalating delay.
• Account lockout or throttling after repeated failures (balanced against denial-of-service — a pure hard-lock lets an attacker lock out real users).
• CAPTCHA / proof-of-work after a threshold.
• Uniform responses so the system never reveals whether an account exists. "Invalid email or password" for both cases; password reset says "if an account exists, we sent a link" regardless. Different messages or timings for existing vs. missing accounts leak a user list.

5. MFA, OTP, and password reset

Reset tokens must be single-use, short-lived, high-entropy, and invalidated once used or once a new one is issued. Delivering OTP over SMS is weak (SIM-swap) but common; note the tradeoff rather than assuming. On any password change or reset, invalidate existing sessions — a reset is often a response to compromise, so leaving old sessions alive defeats the point.

Output format

When reviewing existing code, produce a findings report — not a vague opinion. Use this structure so results are scannable and actionable:

## Auth Review: <component>

### Critical   (exploitable now — fix before shipping)
- [finding] — <what's wrong> → <the fix>

### High       (weakens security materially)
- ...

### Medium / Hardening   (defense in depth)
- ...

### Verified good   (things already done right — say so)
- ...

Each finding names the concrete fix, not just the problem. When designing new auth rather than reviewing, output the implementation plus a short note on which of the five areas above each part addresses.

Proof before done

Consistent with a "no feature is complete without evidence" discipline: an auth fix is not finished until it's demonstrated. For a logout fix, that means showing the token is actually rejected after logout — e.g. call a protected endpoint with the old token and show a 401, not just "the code looks right." Prefer a real test run (unit or integration) over assertion. If the stack has a test runner, write the test; if not, show the request/response evidence.

References

• references/session-patterns.md — Concrete, copyable implementations for stateful sessions, JWT with revocation, and refresh-token rotation. Read it when implementing or fixing a session model rather than guessing the details.
• references/checklist.md — A condensed pre-ship checklist covering all five areas, for a fast final pass before shipping an auth change.