Security Misconfiguration in Fiber with Firestore

Security misconfiguration in a Fiber API that interacts with Firestore often arises from overly permissive Firestore security rules combined with missing validation or authorization checks in the route handlers. When rules allow read or write access based solely on request path parameters or unverified client-supplied document IDs, an unauthenticated or low-privilege caller can access or modify data belonging to other users. This becomes a BOLA/IDOR issue when the API endpoint uses an identifier (such as a user ID or document ID) directly from the request without confirming that the authenticated subject owns that resource.

For example, consider a route like /users/:userId/profile. If the handler builds a Firestore reference using the userId from the URL and retrieves the document without verifying that the requesting subject matches that ID, an attacker can enumerate user IDs and read or update any profile. Misconfigured Firestore rules that use request.auth != null but do not enforce ownership (e.g., checking request.auth.uid against document fields) can allow unauthorized access even when authentication is required. Similarly, rules that enable writes to entire collections or allow updates to sensitive fields without validating operation context can lead to privilege escalation or data exposure.

Another common pattern is using client-supplied query constraints without server-side validation. If an endpoint forwards filter values directly to Firestore (e.g., searching documents by a client-provided field), a misconfigured rule or missing check may expose more data than intended. Because Fiber does not enforce server-side schemas, developers must explicitly validate inputs and enforce least-privilege access in both application logic and Firestore rules. The combination of a flexible NoSQL database and a minimalist web framework increases the risk when security boundaries are not explicitly defined and tested.

To remediate misconfiguration, enforce strict ownership checks and validate all inputs before constructing Firestore queries or document paths. In Fiber, handle authorization in the route handler and ensure Firestore security rules mirror these constraints. Below is a secure example for a /users/:userId/profile endpoint that ensures the authenticated user can only access their own profile.

const { app, json } = require('express');
const { initializeApp } = require('firebase-admin/app');
const { getFirestore, doc, getDoc, updateDoc } = require('firebase-admin/firestore');

initializeApp();
const db = getFirestore();

const express = require('express');
const app = express();
app.use(json());

app.get('/users/:userId/profile', async (req, res) => {
  // In a real app, obtain the subject from middleware (e.g., verified ID token)
  const subjectUid = req.headers['x-subject-uid'];
  const userId = req.params.userId;

  if (!subjectUid) {
    return res.status(401).json({ error: 'Unauthenticated' });
  }
  if (subjectUid !== userId) {
    return res.status(403).json({ error: 'Forbidden: cannot access other profiles' });
  }

  const userRef = doc(db, 'users', userId);
  const snapshot = await getDoc(userRef);
  if (!snapshot.exists()) {
    return res.status(404).json({ error: 'Not found' });
  }
  res.json(snapshot.data());
});

app.patch('/users/:userId/profile', async (req, res) => {
  const subjectUid = req.headers['x-subject-uid'];
  const userId = req.params.userId;
  const updates = req.body;

  if (!subjectUid) {
    return res.status(401).json({ error: 'Unauthenticated' });
  }
  if (subjectUid !== userId) {
    return res.status(403).json({ error: 'Forbidden: cannot update other profiles' });
  }

  // Validate updates to avoid unwanted fields
  if (!updates.displayName || typeof updates.displayName !== 'string') {
    return res.status(400).json({ error: 'Invalid payload' });
  }

  const userRef = doc(db, 'users', userId);
  await updateDoc(userRef, {
    displayName: updates.displayName,
    updatedAt: new Date(),
  });
  res.json({ ok: true });
});

On the Firestore side, ensure security rules enforce ownership and limit write scope. For example:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

Additional remediation steps include validating input types and lengths, avoiding direct passthrough of client values to queries, and using server-side constants for collection names. For broader coverage, use the middleBrick CLI to scan your endpoints and Firestore rules; the middlebrick scan <url> command can surface misconfigured permissions and input validation gaps. If you integrate the GitHub Action, you can fail builds when security scores drop below your chosen threshold, and the Pro plan provides continuous monitoring and compliance mapping to help maintain secure configurations over time.