Storage security rules in Firebase - Step-by-Step Execution

Process Flow - Storage security rules

User sends request

↓

Check authentication?

No→Deny access

Yes↓

Check rules conditions

↓

Allow access

When a user tries to access storage, Firebase checks if they are signed in, then evaluates security rules to allow or deny access.

Execution Sample

Firebase

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

This rule allows only authenticated users to read files and denies all write requests.

Process Table

Step	Request Type	User Authenticated?	Rule Condition	Access Decision
1	Read	Yes	request.auth != null	Allow
2	Write	Yes	false	Deny
3	Read	No	request.auth != null	Deny
4	Write	No	false	Deny

💡 Access is denied if user is not authenticated or rule condition is false.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
request.auth	null	object (user info)	object (user info)	null	null
rule condition	N/A	true	false	false	false
access decision	N/A	Allow	Deny	Deny	Deny

Key Moments - 2 Insights

Why does a read request from an unauthenticated user get denied?

Because the rule requires request.auth != null to allow read, and unauthenticated users have request.auth as null (see execution_table row 3).

Why are all write requests denied even if the user is authenticated?

The write rule condition is false, so no write is allowed regardless of authentication (see execution_table rows 2 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the access decision for a read request by an authenticated user?

A Deny

B Allow

C Depends on file path

D Error

💡 Hint

Check execution_table row 1 under 'Access Decision'

At which step does the condition 'request.auth != null' evaluate to false?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at execution_table rows where User Authenticated? is No

If the write rule changed to 'allow write: if request.auth != null;', what would happen at step 2?

A Error in rules

B Access would still be denied

C Access would be allowed

D Access depends on file path

💡 Hint

Compare current write rule condition false with new condition true for authenticated user

Concept Snapshot

Firebase Storage Security Rules:
- Check if user is authenticated (request.auth != null)
- Allow or deny read/write based on conditions
- Deny access if conditions fail
- Rules apply to file paths
- Always test rules with different user states

Full Transcript

Firebase Storage security rules control who can read or write files. When a user sends a request, Firebase first checks if the user is signed in. If not signed in, access is denied. If signed in, Firebase checks the rule conditions. For example, a rule may allow reads only if the user is authenticated and deny all writes. Each request is evaluated step-by-step to decide if access is allowed or denied. This ensures storage is secure and only authorized users can access files.

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