Storage security rules in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are Firebase Storage security rules?

Firebase Storage security rules control who can read or write files in your storage. They protect your files from unauthorized access.

beginner

How do you allow only authenticated users to upload files in Firebase Storage?

Use the rule:
allow write: if request.auth != null;
This means only users signed in can upload files.

intermediate

What does request.resource.size check in Firebase Storage rules?

It checks the size of the file being uploaded. You can limit file size to prevent very large uploads.

intermediate

Why use resource.metadata in Firebase Storage rules?

It lets you check metadata of existing files, like content type or custom tags, to control access or updates.

advanced

What is a best practice for Firebase Storage security rules?

Start with strict rules denying all access, then allow only what is needed. Always test rules to avoid accidental exposure.

Which Firebase Storage rule allows only signed-in users to read files?

A allow read: if request.time < timestamp.date(2025, 1, 1);

B allow read: if true;

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

D allow read: if resource.size < 1048576;

Only request.auth != null checks if the user is signed in.

How can you limit file uploads to 5 MB in Firebase Storage rules?

A allow write: if request.resource.size < 5242880;

B allow write: if resource.size < 5242880;

C allow write: if request.auth != null;

D allow write: if request.time < timestamp.date(2024, 12, 31);

Use request.resource.size to check the size of the new file being uploaded.

What does resource represent in Firebase Storage rules?

A The file being uploaded

B The existing file in storage

C The user making the request

D The storage bucket

resource refers to the current file already stored.

Which is a safe default rule for Firebase Storage?

A allow read, write: if false;

B allow read, write: if true;

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

D allow write: if request.time < timestamp.date(2025, 1, 1);

Denying all access by default is safest to prevent accidental exposure.

How can you check if a user owns a file in Firebase Storage rules?

A Use <code>allow read: if true;</code>

B Check <code>request.time</code>

C Check <code>request.resource.size</code>

D Compare <code>request.auth.uid</code> with <code>resource.metadata.ownerId</code>

You compare the user's ID with the file's owner metadata to allow access only to owners.

Explain how Firebase Storage security rules control access to files.

Think about who can do what with files and how rules check that.

Describe a simple rule to allow only authenticated users to upload files smaller than 2 MB.

Combine user check and file size limit in one rule.

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