Broken Access Control in Restify with Firestore

Broken Access Control occurs when an API does not properly enforce authorization between subjects and objects. In a Restify service that uses Firestore as the backend, this commonly arises when route handlers reference user identifiers only from client-supplied data (e.g., URL parameters or request bodies) instead of deriving them from the authenticated identity. Firestore security rules can complement application logic, but they are not a substitute for explicit server-side authorization checks. If a Restify endpoint accepts a resource ID such as /users/:userId/profile and directly forwards that ID to Firestore without verifying that the authenticated subject owns that userId, an attacker can enumerate or manipulate IDs to access other users' data.

For example, consider a Restify handler that reads a user document using the ID provided in the route. An attacker can change :userId to another valid user ID and, if the handler does not compare the authenticated UID to the requested ID, the server may return another user’s profile. This is a classic BOLA (Broken Level of Authorization) / IDOR pattern. In Firestore, the effective permissions for a read or write depend on both security rules and the application’s authorization logic; rules alone cannot prevent an application from explicitly passing a different document path based on unchecked input. Additionally, Firestore indexes and flexible data models can expose many collections; without proper per-request authorization, an attacker may leverage mass assignment or over-privileged service accounts used by the Restify backend to escalate privileges or read sensitive fields.

Another vector specific to REST APIs built on Restify is insecure direct object references combined with Firestore’s real-time listeners or queries. If a handler builds a query using concatenated or interpolated identifiers from untrusted sources, an attacker may attempt to inject constraints to retrieve more data than intended. Firestore’s query model does not implicitly restrict scope to the requesting user unless the application explicitly includes uid-based filters. Without such constraints and server-side checks, the API may leak data across tenants. This becomes more impactful when Firestore documents contain sensitive fields (e.g., roles, tokens, PII), and the handler returns entire documents to the client without scrubbing or field-level authorization. The combination of Restify’s route flexibility and Firestore’s rich query capabilities increases the need for strict, per-request authorization tied to the authenticated identity and the principle of least privilege.

To remediate Broken Access Control in a Restify service using Firestore, always derive the subject identifier from the authenticated context rather than from route parameters or request bodies. Use a verified UID from the authentication layer (e.g., a verified JWT or session) to scope all Firestore operations. Below are concrete, realistic code examples for Restify handlers that enforce ownership before reading or writing Firestore documents.

const restify = require('restify');
const { initializeApp, cert } = require('firebase-admin/app');
const { getFirestore } = require('firebase-admin/firestore');

// Initialize Firestore with least-privilege service account
initializeApp({
  credential: cert({
    projectId: process.env.FIREBASE_PROJECT_ID,
    clientEmail: process.env.FIREBASE_CLIENT_EMAIL,
    privateKey: process.env.FIREBASE_PRIVATE_KEY.replace(/\\n/g, '\n'),
  }),
});
const db = getFirestore();

const server = restify.createServer();
server.use(restify.plugins.bodyParser());

// Example: Get user profile — ensure the requesting user matches the document ID
server.get('/users/:uid/profile', async (req, res, next) => {
  const requestingUid = req.authorizedUser ? req.authorizedUser.uid : null; // from auth middleware
  const targetUid = req.params.uid;

  if (!requestingUid) {
    return next(new restify.UnauthorizedError('Missing authentication'));
  }
  if (requestingUid !== targetUid) {
    return next(new restify.ForbiddenError('Access denied to this profile'));
  }

  const docRef = db.collection('users').doc(targetUid);
  const doc = await docRef.get();
  if (!doc.exists) {
    return next(new restify.NotFoundError('Profile not found'));
  }
  // Explicitly pick safe fields instead of returning the entire document
  const data = doc.data();
  res.send({
    uid: doc.id,
    displayName: data.displayName,
    email: data.email,
    // omit sensitive fields such as tokens or roles
  });
  return next();
});

// Example: Update user settings — scope write by authenticated UID
server.patch('/users/:uid/settings', async (req, res, next) => {
  const requestingUid = req.authorizedUser ? req.authorizedUser.uid : null;
  const targetUid = req.params.uid;

  if (!requestingUid || requestingUid !== targetUid) {
    return next(new restify.ForbiddenError('Cannot update other users settings'));
  }

  const settingsRef = db.collection('users').doc(targetUid).collection('settings').doc('prefs');
  await settingsRef.set(req.body, { merge: true });
  res.send({ ok: true });
  return next();
});

// Example: Admin read — verify role via the authenticated identity, not request parameters
server.get('/admin/users/:uid', async (req, res, next) => {
  const requestingUid = req.authorizedUser ? req.authorizedUser.uid : null;
  const requestingAdmin = req.authorizedUser ? req.authorizedUser.isAdmin : false;

  if (!requestingAdmin) {
    return next(new restify.ForbiddenError('Admin access required'));
  }
  // Admins may query any user, but still enforce server-side scoping for clarity
  const docRef = db.collection('users').doc(req.params.uid);
  const doc = await docRef.get();
  if (!doc.exists) {
    return next(new restify.NotFoundError('User not found'));
  }
  res.send(doc.data());
  return next();
});

In these examples, the authenticated identity drives authorization, and Firestore reads/writes are scoped accordingly. Do not rely on Firestore security rules to enforce user ownership when the application path is derived from client-controlled parameters. Combine rules with server-side checks: rules can deny unexpected access as a safety net, but the API must not pass unauthorized document paths. Additionally, avoid returning entire Firestore documents; explicitly select fields needed by the client to reduce exposure of sensitive data and mitigate mass assignment risks. For production, centralize authorization logic (e.g., a verifyRequestScope function) to ensure consistency across endpoints and simplify audits.