Storage policies for access control in Supabase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allow authenticated users to select files from storage.

Supabase

create policy "Allow authenticated read" on storage.objects for select to [1] using (auth.role() = 'authenticated');

Drag options to blanks, or click blank then click option'

Aauthenticated

Bpublic

Canonymous

Dadmin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' allows anyone to access files.

Using 'anonymous' is not a valid role for authenticated users.

The policy must specify the role 'authenticated' to allow only logged-in users to select files.

2fill in blank

medium

Complete the code to allow users to insert files only if they own the file.

Supabase

create policy "Allow insert if owner" on storage.objects for insert to authenticated with check (auth.uid() = [1]);

Drag options to blanks, or click blank then click option'

Aowner_id

Bauth.uid()

Cbucket_id

Dmetadata->>'owner'

Attempts:

3 left

💡 Hint

Common Mistakes

Using bucket_id does not relate to user ownership.

Using auth.uid() makes the condition always true.

The check must compare the authenticated user's ID with the user ID in the request, so metadata->>'owner' is correct.

3fill in blank

hard

Fix the error in the policy to allow users to delete only their own files.

Supabase

create policy "Allow delete own files" on storage.objects for delete to authenticated using (auth.uid() = [1]);

Drag options to blanks, or click blank then click option'

Abucket_id

Bmetadata->>'owner'

Cowner_id

Dauth.uid()

Attempts:

3 left

💡 Hint

Common Mistakes

Using auth.uid() on the right side compares the same value, which is incorrect.

Using bucket_id does not relate to ownership.

The ownership is stored in the file metadata under 'owner', so the check must use metadata->>'owner'.

4fill in blank

hard

Fill both blanks to allow users to update files only if they own them and are authenticated.

Supabase

create policy "Allow update own files" on storage.objects for update to [1] using (auth.uid() = [2]);

Drag options to blanks, or click blank then click option'

Aauthenticated

Bpublic

Cmetadata->>'owner'

Danonymous

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' allows anyone to update files.

Using 'anonymous' is not a valid role for authenticated users.

The policy must apply to authenticated users and check ownership via metadata->>'owner'.

5fill in blank

hard

Fill all three blanks to create a policy that allows authenticated users to select files only from their own bucket and if they own the file.

Supabase

create policy "Allow select own bucket files" on storage.objects for select to [1] using (bucket_id = [2] and auth.uid() = [3]);

Drag options to blanks, or click blank then click option'

Aauthenticated

Bmetadata->>'owner'

Cauth.uid()

D'user_bucket'

Attempts:

3 left

💡 Hint

Common Mistakes

Using auth.uid() directly for bucket_id is incorrect.

Using 'public' role allows anyone to select files.

The policy applies to authenticated users, checks the bucket_id equals 'user_bucket', and verifies ownership via metadata->>'owner'.

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