Authentication-based rules in Firebase - Mini Project: Build & Apply

Firebase Authentication-Based Security Rules

📖 Scenario: You are building a simple Firebase Firestore database for a small app where users can save personal notes. Each user should only be able to read and write their own notes. You will create security rules that check the user's authentication ID to enforce this.

🎯 Goal: Create Firebase Firestore security rules that allow authenticated users to read and write only their own documents in the notes collection. Each document's ID matches the user's UID.

📋 What You'll Learn

Create a notes collection where each document ID is the user's UID

Allow read and write access only if the user is authenticated

Allow read and write access only if the document ID matches the authenticated user's UID

💡 Why This Matters

🌍 Real World

Firebase security rules protect user data by ensuring only authorized users can access or modify their own information.

💼 Career

Understanding authentication-based rules is essential for cloud engineers and developers working with Firebase or similar backend services to secure applications.

1

Define the notes collection in Firestore rules

Write the initial Firestore rules structure with a match block for the notes collection using match /notes/{userId}.

Firebase

// Define Firestore rules with a match block for notes collection
service cloud.firestore {
  match /databases/{database}/documents {
    // Your code here
  }
}

Hint

Start by writing match /notes/{userId} inside the match /databases/{database}/documents block.

2

Add authentication check variable

Inside the match /notes/{userId} block, create a variable called isSignedIn that checks if request.auth != null.

Firebase

service cloud.firestore {
  match /databases/{database}/documents {
    match /notes/{userId} {
      // Define isSignedIn variable here
      // Your code here
    }
  }
}

Hint

Use let isSignedIn = request.auth != null; to check if the user is signed in.

3

Add rule to allow access only if user is authenticated and owns the document

Replace the allow read, write: if false; line with a rule that allows read and write only if isSignedIn is true and request.auth.uid == userId.

Firebase

service cloud.firestore {
  match /databases/{database}/documents {
    match /notes/{userId} {
      // Update allow rule here
      // Your code here
      let isSignedIn = request.auth != null;
    }
  }
}

Hint

Use allow read, write: if isSignedIn && request.auth.uid == userId; to restrict access.

4

Complete the Firestore security rules

Ensure the full Firestore security rules include the service cloud.firestore block, the match /databases/{database}/documents block, and the match /notes/{userId} block with the authentication-based access rule.

Firebase

service cloud.firestore {
  match /databases/{database}/documents {
    match /notes/{userId} {
      // Finalize rules here
      // Your code here
      let isSignedIn = request.auth != null;
      allow read, write: if isSignedIn && request.auth.uid == userId;
    }
  }
}

Hint

Make sure all parts of the rules are included and correctly nested.

Practice

1. What does the Firebase rule condition request.auth != null check for?

easy

A. If the request is from a mobile device

B. If the user has admin privileges

C. If the user is signed in

D. If the database is empty

Solution

1. Step 1: Understand the meaning of request.auth

   request.auth holds authentication info if a user is signed in; otherwise, it is null.

2. Step 2: Interpret the condition request.auth != null

   This condition is true only when a user is signed in, meaning authentication info exists.

3. Final Answer:

   If the user is signed in -> Option C

4. Quick Check:

   Authentication check = user signed in [OK]

Hint: Check if request.auth is not null to confirm user login [OK]

Common Mistakes:

• Thinking it checks for admin rights
• Assuming it checks device type
• Confusing with database content check

2. Which of the following is the correct Firebase rule syntax to allow read access only to authenticated users?

easy

A. allow read: if request.auth == true;

B. allow read: if request.auth == null;

C. allow read: if request.auth.id == null;

D. allow read: if request.auth != null;

Solution

1. Step 1: Identify the correct condition for authenticated users

   The condition request.auth != null means the user is signed in.

2. Step 2: Match the syntax with the correct condition

   allow read: if request.auth != null; uses allow read: if request.auth != null; which correctly allows reads only for signed-in users.

3. Final Answer:

   allow read: if request.auth != null; -> Option D

4. Quick Check:

   Correct syntax for auth check = allow read: if request.auth != null; [OK]

Hint: Use != null to allow only signed-in users [OK]

Common Mistakes:

• Using == null which allows unauthenticated users
• Checking request.auth.id instead of request.auth
• Using boolean true instead of null check

3. Given this Firebase rule snippet:

allow read: if request.auth != null && request.auth.uid == resource.data.ownerId;

What happens if a signed-in user tries to read data where ownerId is different from their uid?

medium

A. Read access is denied

B. Read access is granted only if user is admin

C. Read access is granted

D. Read access causes a syntax error

Solution

1. Step 1: Analyze the rule condition

   The rule requires the user to be signed in and their user ID (uid) to match the data's ownerId.

2. Step 2: Evaluate the scenario where uid differs from ownerId

   Since the IDs do not match, the condition fails, so access is denied.

3. Final Answer:

   Read access is denied -> Option A

4. Quick Check:

   User ID match required = access denied if no match [OK]

Hint: User ID must match ownerId for access [OK]

Common Mistakes:

• Assuming any signed-in user can read
• Thinking admin rights are checked here
• Confusing syntax error with logic denial

4. Identify the error in this Firebase rule:

allow write: if request.auth.uid = resource.data.ownerId;

medium

A. Missing check for request.auth != null

B. Using single equals (=) instead of double equals (==)

C. Incorrect field name 'ownerId'

D. No error, rule is correct

Solution

1. Step 1: Check the comparison operator

   The rule uses a single equals = which is an assignment, not a comparison.

2. Step 2: Correct operator usage in Firebase rules

   Firebase rules require double equals == for equality checks.

3. Final Answer:

   Using single equals (=) instead of double equals (==) -> Option B

4. Quick Check:

   Use == for comparisons, not = [OK]

Hint: Use == for equality, not = [OK]

Common Mistakes:

• Confusing assignment (=) with comparison (==)
• Ignoring null check for request.auth
• Assuming field names cause error

5. You want to write a Firebase rule that allows users to update their own profile only if they are signed in and the profile's userId matches their uid. Which rule correctly implements this?

hard

A. allow update: if request.auth != null && request.auth.uid == resource.data.userId;

B. allow update: if request.auth != null && request.auth.uid == request.resource.data.userId;

C. allow update: if request.auth == null || request.auth.uid != resource.data.userId;

D. allow update: if request.auth != null && request.auth.uid == resource.data.ownerId;

Solution

1. Step 1: Understand the difference between resource.data and request.resource.data

   resource.data is the existing data; request.resource.data is the new data being written.

2. Step 2: Match user ID with existing data's userId during update

   To allow update only if the profile's existing userId matches the user's uid, compare request.auth.uid with resource.data.userId.

3. Step 3: Verify the condition

   allow update: if request.auth != null && request.auth.uid == resource.data.userId; correctly checks the user is signed in and owns the existing profile.

4. Final Answer:

   allow update: if request.auth != null && request.auth.uid == resource.data.userId; -> Option A

5. Quick Check:

   Check existing data userId matches auth.uid for update [OK]

Hint: Use resource.data for existing ownership checks [OK]

Common Mistakes:

• Using request.resource.data (new data) instead of resource.data (existing data)
• Allowing updates without authentication
• Using wrong field names for user ID