Auth Security Agent Skill

import crypto from 'crypto';

// 1. Generate code verifier (43-128 chars)
function generateCodeVerifier(): string {
  return crypto.randomBytes(32).toString('base64url');
}

// 2. Generate code challenge
function generateCodeChallenge(verifier: string): string {
  return crypto
    .createHash('sha256')
    .update(verifier)
    .digest('base64url');
}

// 3. Authorization request
const verifier = generateCodeVerifier();
const challenge = generateCodeChallenge(verifier);

const authUrl = new URL('https://auth.example.com/authorize');
authUrl.searchParams.set('response_type', 'code');
authUrl.searchParams.set('client_id', CLIENT_ID);
authUrl.searchParams.set('redirect_uri', REDIRECT_URI);
authUrl.searchParams.set('code_challenge', challenge);
authUrl.searchParams.set('code_challenge_method', 'S256');
authUrl.searchParams.set('scope', 'openid profile email');
authUrl.searchParams.set('state', generateState());

// 4. Token exchange (after redirect)
const tokenResponse = await fetch('https://auth.example.com/token', {
  method: 'POST',
  headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
  body: new URLSearchParams({
    grant_type: 'authorization_code',
    code: authorizationCode,
    redirect_uri: REDIRECT_URI,
    client_id: CLIENT_ID,
    code_verifier: verifier, // Prove we initiated the request
  }),
});

// Recommended: ES256
import { SignJWT, jwtVerify } from 'jose';

const privateKey = await importPKCS8(PRIVATE_KEY_PEM, 'ES256');
const publicKey = await importSPKI(PUBLIC_KEY_PEM, 'ES256');

// Sign
const token = await new SignJWT({ sub: userId, scope: 'read write' })
  .setProtectedHeader({ alg: 'ES256', typ: 'JWT', kid: keyId })
  .setIssuer('https://auth.example.com')
  .setAudience('https://api.example.com')
  .setExpirationTime('15m')
  .setIssuedAt()
  .setJti(crypto.randomUUID())
  .sign(privateKey);

interface AccessTokenPayload {
  // Standard claims
  iss: string;  // Issuer
  sub: string;  // Subject (user ID)
  aud: string;  // Audience
  exp: number;  // Expiration (Unix timestamp)
  iat: number;  // Issued at
  jti: string;  // JWT ID (unique identifier)

  // Custom claims
  scope: string;      // Permissions
  email?: string;     // User email
  roles?: string[];   // User roles
}

import { jwtVerify, errors } from 'jose';

async function verifyAccessToken(token: string): Promise<AccessTokenPayload> {
  try {
    const { payload } = await jwtVerify(token, publicKey, {
      // CRITICAL: Explicitly specify allowed algorithms
      algorithms: ['ES256'],

      // Validate standard claims
      issuer: 'https://auth.example.com',
      audience: 'https://api.example.com',

      // Clock tolerance for sync issues
      clockTolerance: 30,
    });

    // Additional validation
    if (!payload.scope?.includes('read')) {
      throw new Error('Insufficient scope');
    }

    return payload as AccessTokenPayload;
  } catch (err) {
    if (err instanceof errors.JWTExpired) {
      throw new AuthError('Token expired', 'TOKEN_EXPIRED');
    }
    if (err instanceof errors.JWTClaimValidationFailed) {
      throw new AuthError('Invalid token claims', 'INVALID_CLAIMS');
    }
    throw new AuthError('Invalid token', 'INVALID_TOKEN');
  }
}

// Set token in HttpOnly cookie (server-side)
function setAuthCookie(res: Response, token: string) {
  res.cookie('access_token', token, {
    httpOnly: true,     // Not accessible via JavaScript
    secure: true,       // HTTPS only
    sameSite: 'strict', // CSRF protection
    maxAge: 15 * 60 * 1000, // 15 minutes
    path: '/api',       // Only sent to API routes
  });
}

// Refresh token (longer-lived)
function setRefreshCookie(res: Response, token: string) {
  res.cookie('refresh_token', token, {
    httpOnly: true,
    secure: true,
    sameSite: 'strict',
    maxAge: 7 * 24 * 60 * 60 * 1000, // 7 days
    path: '/api/auth/refresh',  // Only for refresh endpoint
  });
}

// Store in memory (NOT localStorage/sessionStorage)
class TokenManager {
  private accessToken: string | null = null;

  setToken(token: string) {
    this.accessToken = token;
  }

  getToken(): string | null {
    return this.accessToken;
  }

  clearToken() {
    this.accessToken = null;
  }
}

// Use with Refresh Token Rotation
// Refresh token in HttpOnly cookie
// Access token in memory

1. Client sends refresh_token
2. Server validates refresh_token
3. Server generates NEW access_token + NEW refresh_token
4. Server INVALIDATES old refresh_token
5. Server returns new tokens
6. Client stores new tokens

async function refreshTokens(refreshToken: string) {
  // Find token in database
  const stored = await db.refreshToken.findUnique({
    where: { token: hashToken(refreshToken) },
    include: { user: true },
  });

  if (!stored) {
    throw new AuthError('Invalid refresh token', 'INVALID_TOKEN');
  }

  // Check if already used (reuse detection)
  if (stored.usedAt) {
    // Potential token theft - revoke ALL user tokens
    await db.refreshToken.deleteMany({
      where: { userId: stored.userId },
    });

    // Alert security team
    await alertSecurityTeam({
      event: 'REFRESH_TOKEN_REUSE',
      userId: stored.userId,
      tokenId: stored.id,
    });

    throw new AuthError('Token reuse detected', 'TOKEN_REUSE');
  }

  // Check expiration
  if (stored.expiresAt < new Date()) {
    throw new AuthError('Refresh token expired', 'TOKEN_EXPIRED');
  }

  // Mark as used (but keep for reuse detection)
  await db.refreshToken.update({
    where: { id: stored.id },
    data: { usedAt: new Date() },
  });

  // Generate new tokens
  const newAccessToken = await generateAccessToken(stored.user);
  const newRefreshToken = await generateRefreshToken(stored.user);

  // Store new refresh token
  await db.refreshToken.create({
    data: {
      token: hashToken(newRefreshToken),
      userId: stored.userId,
      expiresAt: addDays(new Date(), 7),
      previousTokenId: stored.id, // Chain for audit
    },
  });

  return {
    accessToken: newAccessToken,
    refreshToken: newRefreshToken,
  };
}

// WRONG: Trusts header algorithm
jwt.verify(token, key); // Uses alg from header

// CORRECT: Explicit algorithm
jwt.verify(token, key, { algorithms: ['ES256'] });

// Use SameSite cookies
res.cookie('session', token, {
  sameSite: 'strict', // or 'lax' for cross-site links
});

// Or double-submit cookie pattern
const csrfToken = crypto.randomBytes(32).toString('hex');
res.cookie('csrf', csrfToken, { httpOnly: false });
// Client sends csrf token in header

// Content Security Policy
res.setHeader('Content-Security-Policy', [
  "default-src 'self'",
  "script-src 'self'",
  "style-src 'self' 'unsafe-inline'",
].join('; '));

// Use HttpOnly cookies for tokens
// Never store tokens in localStorage

// Demonstration of Proof of Possession
// Bind token to client's key pair

const dpopProof = await new SignJWT({
  htm: 'POST',
  htu: 'https://api.example.com/resource',
  ath: await hashAccessToken(accessToken), // Access token hash
})
  .setProtectedHeader({ alg: 'ES256', typ: 'dpop+jwt', jwk: publicKey })
  .setJti(crypto.randomUUID())
  .setIssuedAt()
  .sign(privateKey);

// Send with request
fetch('https://api.example.com/resource', {
  headers: {
    Authorization: `DPoP ${accessToken}`,
    DPoP: dpopProof,
  },
});

// Revoke all user tokens (e.g., password change, logout all)
async function revokeAllUserTokens(userId: string) {
  await db.refreshToken.deleteMany({
    where: { userId },
  });

  // If using token blacklist for access tokens
  await redis.sadd(`revoked:${userId}`, Date.now());
  await redis.expire(`revoked:${userId}`, 15 * 60); // 15 min (access token lifetime)
}

// Check blacklist during verification
async function isTokenRevoked(userId: string, iat: number): Promise<boolean> {
  const revokedAt = await redis.get(`revoked:${userId}`);
  return revokedAt && parseInt(revokedAt) > iat * 1000;
}

## OAuth 2.1
- [ ] Using Authorization Code flow
- [ ] PKCE enabled for all clients
- [ ] No implicit or password grants
- [ ] Redirect URI exact matching

## JWT
- [ ] Using ES256 or EdDSA algorithm
- [ ] Explicit algorithm verification
- [ ] Short expiration (≤15 min)
- [ ] Unique jti for each token
- [ ] Issuer and audience validation

## Tokens
- [ ] HttpOnly cookies for web apps
- [ ] Refresh token rotation enabled
- [ ] Reuse detection implemented
- [ ] Token revocation mechanism

## Security
- [ ] HTTPS everywhere
- [ ] SameSite cookies
- [ ] CSP headers configured
- [ ] Rate limiting on auth endpoints
- [ ] Brute force protection