authentication— agent skill

Authentication

Use established auth libraries. Don't build your own. Enforce strong defaults.

Related: access-control, secrets-management, security-context

Rule 1: Never Roll Your Own Auth

Use your framework's auth system. It handles edge cases you haven't thought of.

// WRONG — custom password check
if (user.password === inputPassword) { login(); }

// RIGHT — use bcrypt/argon2 with established library
const valid = await bcrypt.compare(inputPassword, user.passwordHash);
if (valid) { createSession(user); }

Rule 2: Hash Passwords Properly

Use bcrypt, argon2, or scrypt. Never MD5, SHA-1, or SHA-256 alone for passwords.

# WRONG — fast hash, no salt
password_hash = hashlib.sha256(password.encode()).hexdigest()

# RIGHT — slow hash with salt
password_hash = bcrypt.hashpw(password.encode(), bcrypt.gensalt(rounds=12))

Rule 3: Store Tokens in HttpOnly Cookies

Never store auth tokens in localStorage or sessionStorage. They're accessible via XSS.

// WRONG — accessible to any script on the page
localStorage.setItem('token', jwt);

// RIGHT — HttpOnly cookie, not accessible via JavaScript
res.cookie('session', token, {
  httpOnly: true,
  secure: true,
  sameSite: 'strict',
  maxAge: 3600000
});

Rule 4: Implement Account Lockout

Prevent brute force attacks. Lock after repeated failures.

// WRONG — unlimited login attempts
if (!validPassword) return res.status(401).send('Wrong password');

// RIGHT — track failures, lock account
if (user.failedAttempts >= 5) {
  return res.status(423).send('Account locked. Try again in 15 minutes.');
}

Rule 5: Secure Password Reset

Reset tokens must be single-use, time-limited, and random.

// WRONG — predictable token
const token = user.id + Date.now();

// RIGHT — cryptographically random, expires in 1 hour
const token = crypto.randomBytes(32).toString('hex');
await saveResetToken(user.id, token, Date.now() + 3600000);

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