Storage policies for access control in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of storage policies in Supabase?

Storage policies control who can access or modify files in Supabase storage buckets. They help keep data safe by setting rules for reading and writing files.

beginner

How do you define a storage policy in Supabase?

You write a policy using SQL-like syntax that specifies conditions under which users can perform actions like SELECT (read) or INSERT (upload) on storage objects.

intermediate

What does the 'auth.uid()' function do in Supabase storage policies?

It returns the unique ID of the currently logged-in user, allowing policies to check if the user owns the file or has permission to access it.

beginner

Why is it important to restrict public access in storage policies?

Restricting public access prevents unauthorized users from reading or modifying files, protecting sensitive data and user privacy.

intermediate

Give an example condition to allow users to read only their own files in Supabase storage.

A condition like: "bucket_id = 'user-files' AND auth.uid() = metadata->>'user_id'" ensures users can only access files tagged with their own user ID.

What does a storage policy in Supabase control?

A The color of the storage interface

B How fast files upload

C The size limit of the database

D Who can read or write files in storage buckets

Storage policies define access rules for reading and writing files in Supabase storage.

Which function helps identify the current user in a Supabase storage policy?

A auth.uid()

B current_user()

C user_id()

D get_user()

auth.uid() returns the unique ID of the logged-in user for access checks.

Why should you avoid making storage buckets publicly readable?

A To reduce storage costs

B To speed up file downloads

C To protect sensitive files from unauthorized access

D To allow anyone to upload files

Public read access can expose private data, so restricting it improves security.

In a storage policy, what does the condition "auth.uid() = metadata->>'user_id'" mean?

A User can access files they own

B User can access all files

C User cannot access any files

D User can only upload files

This condition allows users to access only files tagged with their own user ID.

Which action does the 'SELECT' keyword control in Supabase storage policies?

A Uploading files

B Reading files

C Deleting files

D Renaming files

'SELECT' controls permission to read or download files from storage.

Explain how storage policies help secure files in Supabase storage.

Think about who can see or change files and how policies set rules for that.

Describe a simple policy condition that allows users to only read their own files.

Focus on matching user identity with file ownership.

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