Insecure Design in Fiber with Firestore

Insecure Design in a Fiber application that uses Firestore typically arises from trusting client-supplied identifiers and misapplying Firestore security rules. When endpoints are designed without enforcing per-request authorization checks, an attacker can manipulate document IDs or query parameters to access or modify data that should be isolated to other users.

Consider a route that retrieves a user document by ID directly from the URL path. If the route does not cross-reference the authenticated user’s ID with the requested document ID, the endpoint exposes a Broken Level of Authorization (BOLA) / Insecure Direct Object Reference (IDOR) pattern. Firestore rules alone are not sufficient if the API layer does not enforce ownership; a misconfigured rule set might allow read access based on document path alone, and an attacker iterating through IDs can enumerate sensitive records.

Another insecure design pattern involves accepting Firestore document paths or keys from the client without validating that the authenticated subject has rights to that exact path. For example, allowing a client to specify a full document path such as users/{uid}/data/{docId} and then passing that directly to Firestore reads or writes can lead to privilege escalation if the backend does not recompute paths from the authenticated identity rather than trusting the client-supplied value.

Additionally, Firestore’s flexible data model can encourage designs where sensitive fields (e.g., roles, administrative flags, or PII) are stored in the same document as user data. If these fields are returned by default in API responses without field-level authorization checks, they may be unintentionally exposed. This becomes a Data Exposure issue when responses include elevated privileges or sensitive metadata that should be restricted to specific roles or administrative interfaces.

The combination of Fiber’s lightweight routing and Firestore’s real-time, schema-less documents can inadvertently encourage shortcuts in authorization logic. Without explicit checks on every request, the API surface may allow horizontal privilege escalation (accessing another user’s data) or vertical escalation (accessing admin-only operations). These design issues map to OWASP API Top 10 controls such as broken object-level authorization and excessive data exposure, and they may also affect compliance mappings for PCI-DSS and SOC2 if sensitive data is not properly isolated and surfaced only to authorized consumers.

Remediation centers on rederiving Firestore paths from authenticated identity and validating ownership on every request. Do not rely on client-supplied document IDs alone; instead, compute document references server-side using the authenticated user’s subject identifier.

Example: a secure read endpoint that fetches a user profile document by authenticated UID, rather than by client-supplied ID.

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

const app = express();
initializeApp();
const db = getFirestore();

app.get('/api/user/profile', async (req, res) => {
  // Assume req.user contains authenticated identity from middleware
  const uid = req.user.uid;
  if (!uid) {
    return res.status(401).json({ error: 'Unauthorized' });
  }

  const docRef = db.collection('users').doc(uid);
  const doc = await docRef.get();

  if (!doc.exists) {
    return res.status(404).json({ error: 'Not found' });
  }

  // Explicitly filter sensitive fields before returning
  const data = doc.data();
  const safePayload = {
    displayName: data.displayName,
    email: data.email,
    // Do not include isAdmin or role unless required and verified
  };
  res.json(safePayload);
});

Example: a secure write/update endpoint that ensures the authenticated user can only update their own document, and recomputes the path instead of using client input as the document key.

app.patch('/api/user/profile', async (req, res) => {
  const uid = req.user.uid;
  if (!uid) {
    return res.status(401).json({ error: 'Unauthorized' });
  }

  const updates = req.body;
  // Do not allow clients to override uid or role unless explicitly authorized
  if (updates.hasOwnProperty('uid') || updates.hasOwnProperty('role') || updates.hasOwnProperty('isAdmin')) {
    return res.status(400).json({ error: 'Invalid fields' });
  }

  const docRef = db.collection('users').doc(uid);
  await docRef.update(updates);
  res.json({ message: 'updated' });
});

For endpoints that must reference related resources, validate ownership explicitly. For example, if a user can manage their own posts stored under users/{uid}/posts/{postId}, recompute the base path from the authenticated UID and ensure the provided postId belongs to that user.

app.get('/api/users/:uid/posts/:postId', async (req, res) => {
  const authenticatedUid = req.user.uid;
  const requestedUid = req.params.uid;

  if (authenticatedUid !== requestedUid) {
    return res.status(403).json({ error: 'Forbidden: cannot access other users posts' });
  }

  const docRef = db.collection('users').doc(requestedUid).collection('posts').doc(req.params.postId);
  const doc = await docRef.get();

  if (!doc.exists) {
    return res.status(404).json({ error: 'Post not found' });
  }

  res.json(doc.data());
});

Apply Firestore security rules that mirror these server-side checks, but never rely on rules alone to enforce authorization at the API layer. Combine runtime validation with rule constraints to reduce risk of misconfiguration. These design changes mitigate BOLA/IDOR and Data Exposure by ensuring every read, write, and delete is scoped to the authenticated subject and validated server-side before touching Firestore.