Storage security rules in Firebase - Mini Project: Build & Apply

Firebase Storage Security Rules Setup

📖 Scenario: You are building a simple app where users can upload and view their own photos securely. You want to make sure that each user can only upload and read their own files in Firebase Storage.

🎯 Goal: Create Firebase Storage security rules that allow authenticated users to read and write only their own files under their user ID folder.

📋 What You'll Learn

Create a variable to represent the user's ID in the path

Add a condition to allow read and write only if the user is authenticated

Add a condition to allow access only to files under the user's own folder

Complete the Firebase Storage rules with proper match and allow statements

💡 Why This Matters

🌍 Real World

Securing user files in Firebase Storage is essential for apps that handle personal data like photos or documents.

💼 Career

Understanding and writing Firebase Storage security rules is a key skill for cloud developers and backend engineers working with Firebase.

1

Define the user ID variable in the path

Create a match block for the path /user_uploads/{userId}/{allPaths=**} and define the variable userId to represent the user folder.

Firebase

service firebase.storage {
  match /b/{bucket}/o {
    // Your code here
  }
}

Hint

Use match to specify the folder path with {userId} as a variable.

2

Add authentication check

Inside the match /user_uploads/{userId}/{allPaths=**} block, add the allow read, write rule with the condition request.auth != null && request.auth.uid == userId.

Firebase

service firebase.storage {
  match /b/{bucket}/o {
    match /user_uploads/{userId}/{allPaths=**} {
      // Your code here
    }
  }
}

Hint

Use request.auth != null to check authentication and compare request.auth.uid with userId.

3

Add read and write rules for user files

Write the allow read, write rule inside the match /user_uploads/{userId}/{allPaths=**} block that permits access only if the user is authenticated and their UID matches userId.

Firebase

service firebase.storage {
  match /b/{bucket}/o {
    match /user_uploads/{userId}/{allPaths=**} {
      // Your code here
    }
  }
}

Hint

This rule ensures only the authenticated user can access their own files.

4

Complete the Firebase Storage security rules

Ensure the entire Firebase Storage security rules file includes the service firebase.storage declaration, the match /b/{bucket}/o block, and the match /user_uploads/{userId}/{allPaths=**} block with the allow read, write rule as specified.

Firebase

service firebase.storage {
  match /b/{bucket}/o {
    match /user_uploads/{userId}/{allPaths=**} {
      // Your code here
    }
  }
}

Hint

Check that all parts of the rules file are present and correctly structured.

Practice

1. What is the main purpose of Firebase Storage security rules?

easy

A. To control who can read or write files in storage

B. To speed up file uploads

C. To increase storage space automatically

D. To backup files to another server

Solution

1. Step 1: Understand the role of security rules

   Security rules define who can access files and what actions they can perform.

2. Step 2: Identify the main function

   The main function is to control read and write permissions to protect data.

3. Final Answer:

   To control who can read or write files in storage -> Option A

4. Quick Check:

   Security rules = control access [OK]

Hint: Security rules control access permissions only [OK]

Common Mistakes:

• Thinking rules speed up uploads
• Believing rules increase storage size
• Confusing rules with backup features

2. Which of the following is the correct syntax to allow only authenticated users to read files in Firebase Storage rules?

easy

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

B. allow read: if user.authenticated == true;

C. allow read: if auth.user != null;

D. allow read: if request.user.authenticated;

Solution

1. Step 1: Check Firebase Storage rules syntax

   The correct way to check authentication is using request.auth != null.

2. Step 2: Compare options

   Only allow read: if request.auth != null; uses the correct Firebase syntax for authentication check.

3. Final Answer:

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

4. Quick Check:

   Authentication check = request.auth != null [OK]

Hint: Use request.auth to check user authentication [OK]

Common Mistakes:

• Using incorrect variable names like user.authenticated
• Missing the 'request.' prefix
• Using wrong boolean checks

3. Given the following Firebase Storage rule snippet, what happens when an unauthenticated user tries to read a file?

service firebase.storage {
  match /b/{bucket}/o {
    match /files/{fileId} {
      allow read: if request.auth != null;
      allow write: if false;
    }
  }
}

medium

A. The read request causes a syntax error

B. The read request is allowed because write is false

C. The read request is allowed because no write permission is needed

D. The read request is denied because the user is not authenticated

Solution

1. Step 1: Analyze the read rule condition

   The rule allows read only if request.auth != null, meaning user must be authenticated.

2. Step 2: Consider unauthenticated user

   An unauthenticated user has request.auth == null, so the condition fails and read is denied.

3. Final Answer:

   The read request is denied because the user is not authenticated -> Option D

4. Quick Check:

   Unauthenticated read = denied [OK]

Hint: Read allowed only if request.auth is not null [OK]

Common Mistakes:

• Ignoring the read condition and assuming access
• Confusing read and write permissions
• Thinking false write affects read

4. Identify the error in this Firebase Storage rule that aims to allow users to write only to their own folder:

service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/{allPaths=**} {
      allow write: if request.auth.uid == userId;
    }
  }
}

medium

A. The variable userId is not defined in the path

B. The wildcard {allPaths=**} is incorrectly used

C. The rule does not allow read access

D. The condition should use request.auth.userId instead of request.auth.uid

Solution

1. Step 1: Check path variables

   The path correctly defines {userId} and {allPaths=**} wildcards.

2. Step 2: Review condition and permissions

   The condition correctly compares request.auth.uid with userId for write permission.

3. Step 3: Identify missing read permission

   The rule only allows write, no read permission is granted, which may block legitimate reads.

4. Final Answer:

   The rule does not allow read access -> Option C

5. Quick Check:

   Missing read permission = problem [OK]

Hint: Check if both read and write permissions are set [OK]

Common Mistakes:

• Assuming write permission covers read
• Confusing request.auth.uid with userId variable
• Misusing wildcard syntax

5. You want to allow users to upload images only to their own folder and only if the file size is less than 2MB. Which Firebase Storage rule correctly enforces this?

service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/{fileName} {
      allow write: if request.auth.uid == userId && request.resource.size < 2 * 1024 * 1024;
      allow read: if request.auth.uid == userId;
    }
  }
}

hard

A. The file size check should use resource.size instead of request.resource.size

B. This rule correctly enforces user ownership and file size limit

C. The rule should allow write without checking user ID

D. The size limit should be checked on the existing file, not request.resource

Solution

1. Step 1: Verify user ownership condition

   The rule checks request.auth.uid == userId to ensure users write only to their folder.

2. Step 2: Verify file size condition

   The rule uses request.resource.size < 2 * 1024 * 1024 to limit uploads to less than 2MB.

3. Step 3: Confirm read permission

   Read is allowed only for the authenticated user matching userId.

4. Final Answer:

   This rule correctly enforces user ownership and file size limit -> Option B

5. Quick Check:

   User ID and size check = correct [OK]

Hint: Use request.resource.size for new file size check [OK]

Common Mistakes:

• Using resource.size instead of request.resource.size
• Not checking user ID for write
• Checking size on existing file instead of upload