🔥 Part 6: Firebase Misconfigurations in Flutter — How Open Rules Can Expose All Your App's Data

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Firebase is often the go-to backend for Flutter apps — it's fast, scalable, and integrates seamlessly. But with great simplicity comes…

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Firebase is often the go-to backend for Flutter apps — it's fast, scalable, and integrates seamlessly. But with great simplicity comes great risk.

In this article, we dive into one of the most widespread security flaws in Flutter apps: Firebase misconfiguration. Specifically, we'll cover how open rules, anonymous auth, and weak client-side logic can lead to complete data exposure — often without even needing to authenticate.

🚨 What's the Risk?

Firebase uses a rules-based system to determine who can read and write to your database, Firestore, and storage buckets.

The problem? Many apps ship with rules like this:

{
  "rules": {
    ".read": "true",
    ".write": "true"
  }
}

Which means anyone on the internet can access or overwrite your app's entire database — no login required.

🕵️‍♂️ Real-World Exploit Scenarios

💣 Exploit 1: Unauthenticated Database Dump

A malicious actor runs:

curl https://your-app.firebaseio.com/.json

And downloads your entire user base, transactions, or chat history.

🧙 Exploit 2: Write Arbitrary Data

Using the same open rules:

curl -X PUT -d '{"isAdmin": true}' https://your-app.firebaseio.com/users/12345/role.json

The attacker now makes themselves an admin.

🕳 Exploit 3: Anonymous Auth Abuse & UID Collision

Many apps allow signInAnonymously() without checks:

await FirebaseAuth.instance.signInAnonymously();

If your rules trust any authenticated user, this becomes an open door:

{
  "rules": {
    ".read": "auth != null",
    ".write": "auth != null"
  }
}

The attacker signs in anonymously and gains full access.

Some apps even bind critical data to UID only, which attackers can guess or brute-force, leading to Insecure Direct Object Reference (IDOR).

📉 Real-World Impact

Risk Level: CRITICAL

🛡️ How to Fix It

✅ Fix 1: Set Secure Firebase Rules

Replace open rules with role-based access control:

{
  "rules": {
    "users": {
      "$uid": {
        ".read": "auth != null && auth.uid == $uid",
        ".write": "auth != null && auth.uid == $uid"
      }
    }
  }
}

✅ Fix 2: Restrict Anonymous Access

If you use signInAnonymously(), gate critical actions:

if (FirebaseAuth.instance.currentUser.isAnonymous) {
  // Restrict sensitive actions
}

Or disable anonymous auth entirely in Firebase Console.

✅ Fix 3: Validate Access on Backend

Never trust client-only logic. Always validate:

• User role/privilege
• Ownership of data
• Auth token expiration

✅ Fix 4: Monitor Firebase Security with Rules Playground

Use the Firebase Rules Simulator to test read/write access:

• Try unauthenticated access
• Try using another user's UID
• Simulate anonymous auth

✅ Fix 5: Log Suspicious Access Patterns

• Use Firebase Cloud Functions to log unusual writes
• Track excessive anonymous account creation
• Alert on permission violations

✅ Secure Firestore Rule Template

service cloud.firestore {
  match /databases/{database}/documents {

    // Users collection
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }

    // Admin-only data
    match /adminData/{doc} {
      allow read, write: if request.auth.token.admin == true;
    }

    // Public content (read-only)
    match /public/{doc} {
      allow read: if true;
      allow write: if false;
    }
  }
}

❌ Anti-Patterns to Avoid

✅ Developer Checklist

• Set secure .read and .write rules in Firebase
• Don't allow open access to sensitive collections
• Disable or restrict anonymous auth
• Enforce UID-based access with server-side validation
• Regularly audit Firebase rules with the simulator
• Log and monitor write operations for anomalies

👀 Up Next

🎯 Part 7: Deep Link Hijacking in Flutter — How Malicious Apps Can Launch Your App in Dangerous States

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