Storage policies for access control in Supabase - Mini Project: Build & Apply

Storage policies for access control

📖 Scenario: You are building a simple file storage system using Supabase. You want to control who can upload and view files based on user authentication.

🎯 Goal: Create storage policies in Supabase to allow only authenticated users to upload files and allow everyone to view files.

📋 What You'll Learn

Create a bucket named user_files

Create a policy to allow authenticated users to upload files to user_files

Create a policy to allow all users to select (view) files from user_files

💡 Why This Matters

🌍 Real World

Storage policies help protect user data by controlling who can upload or view files in cloud storage.

💼 Career

Understanding access control policies is essential for cloud developers and administrators to secure storage resources.

1

Create the storage bucket

Create a storage bucket called user_files using the Supabase CLI or dashboard.

Supabase

# Your code here to create the bucket 'user_files'

Hint

Use the command supabase storage bucket create user_files to create the bucket.

2

Add upload policy for authenticated users

Create a storage policy named Allow authenticated uploads on the user_files bucket that allows users with auth.uid() IS NOT NULL to insert files.

Supabase

supabase storage bucket create user_files
# Your code here to create upload policy for authenticated users

Hint

Use supabase storage policy create with --command insert and --expression "auth.uid() IS NOT NULL".

3

Add view policy for all users

Create a storage policy named Allow public view on the user_files bucket that allows anyone to select files by using the expression true.

Supabase

supabase storage bucket create user_files
supabase storage policy create user_files "Allow authenticated uploads" --command insert --expression "auth.uid() IS NOT NULL"
# Your code here to create view policy for all users

Hint

Use supabase storage policy create with --command select and --expression "true" to allow public viewing.

4

Verify policies are applied

List all storage policies on the user_files bucket to verify that both Allow authenticated uploads and Allow public view policies exist.

Supabase

supabase storage bucket create user_files
supabase storage policy create user_files "Allow authenticated uploads" --command insert --expression "auth.uid() IS NOT NULL"
supabase storage policy create user_files "Allow public view" --command select --expression "true"
# Your code here to list policies on 'user_files'

Hint

Use supabase storage policy list user_files to see all policies on the bucket.

Practice

1. What is the main purpose of storage policies in Supabase?

easy

A. To increase storage space automatically

B. To speed up file uploads

C. To control who can read, upload, or delete files

D. To backup files to another server

Solution

1. Step 1: Understand storage policies

   Storage policies define rules about who can access or change files.

2. Step 2: Identify the main goal

   The main goal is to control access like reading, uploading, or deleting files to keep data safe.

3. Final Answer:

   To control who can read, upload, or delete files -> Option C

4. Quick Check:

   Storage policies = Access control [OK]

Hint: Storage policies = who can do what with files [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax to allow only authenticated users to read files in Supabase storage policy?

easy

A. allow read if user.is_authenticated

B. using (auth.uid() != null)

C. check auth.user == true

D. permit read where auth.id exists

Solution

1. Step 1: Recall Supabase policy syntax

   Supabase uses SQL-like expressions with functions like auth.uid() to check user identity.

2. Step 2: Identify correct condition for authenticated users

   auth.uid() returns user ID if logged in, so checking auth.uid() != null allows only authenticated users.

3. Final Answer:

   using (auth.uid() != null) -> Option B

4. Quick Check:

   auth.uid() != null = authenticated user check [OK]

Hint: Use auth.uid() to check if user is logged in [OK]

Common Mistakes:

• Using incorrect property names like user.is_authenticated
• Writing non-SQL syntax like permit or allow keywords
• Checking auth.id instead of auth.uid()

3. Given this storage policy snippet:

using (bucket_id = 'public' OR auth.uid() = owner_id)

What does this policy allow?

medium

A. Anyone can access files in the 'public' bucket or owners can access their files

B. Only owners can access files in any bucket

C. Only users with bucket_id 'public' can access files

D. No one can access files unless they are owners

Solution

1. Step 1: Analyze the condition

   The condition allows access if bucket_id equals 'public' OR if the user's ID matches owner_id.

2. Step 2: Interpret the meaning

   This means anyone can access files in the 'public' bucket, and owners can access their own files in any bucket.

3. Final Answer:

   Anyone can access files in the 'public' bucket or owners can access their files -> Option A

4. Quick Check:

   bucket_id='public' OR auth.uid()=owner_id = public or owner access [OK]

Hint: OR means either public bucket or owner access allowed [OK]

Common Mistakes:

• Thinking only owners have access
• Assuming bucket_id restricts all access
• Ignoring the OR condition

4. You wrote this policy:

using (auth.uid() = owner_id AND bucket_id = 'private')

But users report they cannot read their private files. What is the likely problem?

medium

A. The policy blocks access because AND requires both conditions, but bucket_id might differ

B. auth.uid() is not a valid function

C. owner_id should be replaced with user_id

D. Policies cannot use AND conditions

Solution

1. Step 1: Understand the AND condition

   AND means both auth.uid() = owner_id and bucket_id = 'private' must be true for access.

2. Step 2: Identify why access fails

   If bucket_id is not exactly 'private' for some files, even owners cannot access them, causing the problem.

3. Final Answer:

   The policy blocks access because AND requires both conditions, but bucket_id might differ -> Option A

4. Quick Check:

   AND requires both true, bucket_id mismatch blocks access [OK]

Hint: AND means all conditions must match exactly [OK]

Common Mistakes:

• Thinking auth.uid() is invalid
• Assuming owner_id is wrong field
• Believing AND is not allowed in policies

5. You want to create a policy that allows users to delete only files they own in any bucket except the 'public' bucket. Which policy condition is correct?

hard

A. using (auth.uid() != owner_id AND bucket_id = 'public')

B. using (auth.uid() = owner_id OR bucket_id != 'public')

C. using (auth.uid() = owner_id AND bucket_id = 'public')

D. using (auth.uid() = owner_id AND bucket_id != 'public')

Solution

1. Step 1: Define the requirements

   Users can delete files only if they own them and the file is not in the 'public' bucket.

2. Step 2: Match the condition

   auth.uid() = owner_id ensures ownership; bucket_id != 'public' excludes the public bucket.

3. Final Answer:

   using (auth.uid() = owner_id AND bucket_id != 'public') -> Option D

4. Quick Check:

   Owner AND not public bucket = correct delete policy [OK]

Hint: Use AND to combine ownership and bucket exclusion [OK]

Common Mistakes:

• Using OR instead of AND allowing wrong deletes
• Checking for equality with 'public' instead of exclusion
• Mixing up != and = operators