dashcaddy/dashcaddy-api/src/security/csrf-protection.js

/**
 * CSRF Protection Module
 * Implements HMAC-signed double-submit cookie pattern for stateless CSRF protection.
 * The cookie contains a random nonce; the header must carry the HMAC signature
 * of that nonce computed with a server-side secret. An attacker who can inject
 * a cookie still cannot forge the matching header without the secret.
 */

const crypto = require('crypto');
const cryptoUtils = require('./crypto-utils');
const { errorResponse } = require('../utils/responses');

const CSRF_TOKEN_LENGTH = 32;
const CSRF_COOKIE_NAME = 'dashcaddy_csrf';
const CSRF_HEADER_NAME = 'x-csrf-token';

/**
 * Generate a cryptographically secure CSRF nonce
 * @returns {string} Base64URL-encoded random nonce
 */
function generateToken() {
  return crypto.randomBytes(CSRF_TOKEN_LENGTH).toString('base64url');
}

/**
 * Compute HMAC signature for a CSRF nonce using the server-side encryption key
 * @param {string} nonce - The random nonce to sign
 * @returns {string} Base64URL-encoded HMAC signature
 */
function signToken(nonce) {
  const key = cryptoUtils.loadOrCreateKey();
  return crypto.createHmac('sha256', key).update(nonce).digest('base64url');
}

/**
 * Parse cookie header string into object
 * @param {string} cookieHeader - Cookie header value
 * @returns {Object} Parsed cookies
 */
function parseCookie(cookieHeader) {
  if (!cookieHeader) return {};

  return cookieHeader.split(';').reduce((cookies, cookie) => {
    const [name, ...rest] = cookie.trim().split('=');
    if (name && rest.length > 0) {
      cookies[name] = rest.join('=');
    }
    return cookies;
  }, {});
}

/**
 * Create CSRF middleware with cookie domain support.
 * When a TLD (e.g. ".sami") is provided, cookies are set with Domain=.sami
 * so they are shared across all subdomains for forward_auth SSO.
 * @param {Object} [options]
 * @param {string} [options.cookieDomain] - e.g. ".sami" to share cookies across subdomains
 * @returns {{ csrfCookieMiddleware: Function, renewCSRFToken: Function }}
 */
function createCSRFMiddleware(options = {}) {
  const { cookieDomain } = options;

  /**
   * Middleware to set CSRF cookie on requests.
   * Preserves existing nonce to avoid invalidating tokens the client has cached.
   * New nonce is generated only on first visit (no cookie) or after TOTP login
   * (which calls renewCSRFToken). If TOTP is disabled, the nonce is set once
   * and never changes.
   */
  function csrfCookieMiddleware(req, res, next) {
    const cookies = parseCookie(req.headers.cookie);
    const existingNonce = cookies[CSRF_COOKIE_NAME];

    // Reuse existing nonce; only generate fresh if no cookie exists yet
    const csrfNonce = existingNonce || generateToken();

    // Store nonce + signature on request so endpoints can access them
    req.csrfToken = signToken(csrfNonce);
    req.csrfNonce = csrfNonce;

    // Only set cookie if it's new (avoids unnecessary Set-Cookie headers)
    if (!existingNonce) {
      const cookieOpts = {
        httpOnly: false, // Must be readable by JavaScript for signing
        secure: req.secure || req.protocol === 'https',
        sameSite: 'strict',
        path: '/',
        maxAge: 365 * 24 * 60 * 60 * 1000 // 1 year (effectively permanent)
      };
      if (cookieDomain) cookieOpts.domain = cookieDomain;
      res.cookie(CSRF_COOKIE_NAME, csrfNonce, cookieOpts);
    }

    next();
  }

  /**
   * Generate a fresh CSRF nonce and set it on the response.
   * Called after TOTP login to rotate the token for the new session.
   * @param {Object} res - Express response object
   * @param {boolean} secure - Whether to set Secure flag on cookie
   * @returns {string} The new CSRF signed token
   */
  function renewCSRFToken(res, secure) {
    const csrfNonce = generateToken();
    const cookieOpts = {
      httpOnly: false,
      secure: !!secure,
      sameSite: 'strict',
      path: '/',
      maxAge: 365 * 24 * 60 * 60 * 1000
    };
    if (cookieDomain) cookieOpts.domain = cookieDomain;
    res.cookie(CSRF_COOKIE_NAME, csrfNonce, cookieOpts);
    return signToken(csrfNonce);
  }

  return { csrfCookieMiddleware, renewCSRFToken };
}

/**
 * Middleware to validate CSRF token on state-changing requests
 * Validates that the token in the cookie matches the token in the header
 */
function csrfValidationMiddleware(req, res, next) {
  const method = req.method.toUpperCase();

  // Skip validation for safe methods
  if (['GET', 'HEAD', 'OPTIONS'].includes(method)) {
    return next();
  }

  // Skip CSRF validation in test environment
  if (process.env.NODE_ENV === 'test') {
    return next();
  }

  // Excluded paths that don't require CSRF validation
  // Note: probe endpoints (/health, /health/live, /health/ready, /healthz,
  // /readyz) are GET-only so they're already excluded by the safe-methods
  // check above. Listed here for explicit safety in case any of them ever
  // accept a POST in the future.
  const excludedPaths = [
    '/api/v1/totp/verify',
    '/api/v1/totp/verify-setup',
    '/api/v1/totp/setup',
    // DC-046 pluggable auth endpoints — public login endpoints, same
    // exemption rationale as the legacy /totp/* paths: a user with no
    // session cookie yet cannot present a CSRF token, so the login flow
    // must be exempt. CSRF protection on the auth boundary is enforced
    // by the SameSite=Lax cookie attribute instead. :provider matches
    // any registered AuthProvider (totp today, email after DC-047).
    '/api/v1/auth/login/:provider/verify',
    '/api/v1/auth/login/:provider/initiate',
    '/api/v1/auth/disable/:provider',
    // DC-048: invite redemption is the same exemption as login verify —
    // the user has no session cookie yet (they just clicked an email link).
    // CSRF on this boundary is enforced by SameSite=Lax instead.
    '/api/v1/auth/invites/:token/accept',
    // DC-053: share-link subscribe + Tailscale redeem originate from the
    // public share page (cross-origin). The token itself is the proof; CSRF
    // is bounded by the token's TTL + scope. Same model as invite accept.
    '/api/v1/share/:token/subscribe',
    '/api/v1/share/:token/redeem-tailscale',
    // DC-055: Stripe Checkout session creation. Browsers hit this from the
    // public pricing page (cross-origin from any *.sami subdomain that
    // serves it); no session cookie exists yet, so a CSRF token can't be
    // anchored. SameSite=Lax on the session cookie doesn't apply (none
    // exists). Threat model: an attacker who can trigger checkout sessions
    // can only force a customer to land on Stripe's hosted page — they
    // can't extract money. Stripe's session id is single-use and tied to a
    // chosen price; reusing it requires Stripe's webhook secret.
    '/api/v1/billing/checkout',
    // DC-057: success page polls this from the customer's browser after
    // Stripe redirects them back. Same CSRF argument as above (no session
    // cookie exists yet) — and the response is the customer's own license
    // code, not anything an attacker can exploit by triggering the lookup.
    '/api/v1/billing/lookup/:sessionId',
    '/health',
    '/health/live',
    '/health/ready',
    '/healthz',
    '/readyz',
    // Machine-to-machine: publishing host POSTs here with its own shared-secret
    // header (X-DashCaddy-Notify-Secret) — browsers never reach this endpoint.
    '/api/v1/system/update-notify'
  ];

  const isExcluded = excludedPaths.some(path => {
    if (req.path === path) return true;
    // Allow `:param` placeholders to match any single segment. Pre-existing
    // bug — literal ':token' never matched real tokens — fixed under DC-053.
    if (path.includes(':')) {
      const pattern = '^' + path.replace(/:[A-Za-z_][A-Za-z0-9_]*/g, '[^/]+') + '$';
      return new RegExp(pattern).test(req.path);
    }
    return false;
  }) || req.path.startsWith('/api/v1/auth/gate/');

  if (isExcluded) {
    return next();
  }

  // Get nonce from cookie
  const cookies = parseCookie(req.headers.cookie);
  const cookieNonce = cookies[CSRF_COOKIE_NAME];

  // Get signed token from header (case-insensitive)
  const headerToken = req.headers[CSRF_HEADER_NAME] ||
                      req.headers[CSRF_HEADER_NAME.toLowerCase()];

  // Skip CSRF for API key-authenticated requests (API keys are not sent automatically by browsers)
  if (req.headers['x-api-key'] || (req.headers.authorization && req.headers.authorization.startsWith('Bearer '))) {
    return next();
  }

  // Validate both values exist
  if (!cookieNonce) {
    console.warn(`[CSRF] Missing CSRF cookie: ${method} ${req.path} from ${req.ip}`);
    return errorResponse(res, 403, '[DC-100] CSRF token missing', {
      message: 'CSRF cookie not found. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }

  if (!headerToken) {
    console.warn(`[CSRF] Missing CSRF header: ${method} ${req.path} from ${req.ip}`);
    return errorResponse(res, 403, '[DC-100] CSRF token missing', {
      message: 'CSRF token not provided in request headers. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }

  // Validate that the header token is the correct HMAC signature of the cookie nonce
  try {
    const expectedSig = signToken(cookieNonce);
    const expectedBuffer = Buffer.from(expectedSig, 'base64url');
    const headerBuffer = Buffer.from(headerToken, 'base64url');

    if (expectedBuffer.length !== headerBuffer.length) {
      throw new Error('Token length mismatch');
    }

    if (!crypto.timingSafeEqual(expectedBuffer, headerBuffer)) {
      throw new Error('Token mismatch');
    }

    // Signature valid — request is authentic
    next();

  } catch (err) {
    console.warn(`[CSRF] Invalid CSRF token: ${method} ${req.path} from ${req.ip} - ${err.message}`);
    return errorResponse(res, 403, '[DC-101] CSRF token invalid', {
      message: 'CSRF token validation failed. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }
}

// Default instance (no domain) for backward compatibility with tests
const defaultInstance = createCSRFMiddleware();

module.exports = {
  CSRF_TOKEN_LENGTH,
  CSRF_COOKIE_NAME,
  CSRF_HEADER_NAME,
  generateToken,
  signToken,
  parseCookie,
  createCSRFMiddleware,
  csrfValidationMiddleware,
  // Default instance exports for backward compat
  csrfCookieMiddleware: defaultInstance.csrfCookieMiddleware,
  renewCSRFToken: defaultInstance.renewCSRFToken
};