Database

Custom Claims & Role-based Access Control (RBAC)


Custom Claims are special attributes attached to a user that you can use to control access to portions of your application. For example:

{
"user_role": "admin",
"plan": "TRIAL",
"user_level": 100,
"group_name": "Super Guild!",
"joined_on": "2022-05-20T14:28:18.217Z",
"group_manager": false,
"items": ["toothpick", "string", "ring"]
}

To implement Role-Based Access Control (RBAC) with custom claims, use a Custom Access Token Auth Hook. This hook runs before a token is issued. You can use it to add additional claims to the user's JWT.

This guide uses the Slack Clone example to demonstrate how to add a user_role claim and use it in your Row Level Security (RLS) policies.

Create a table to track user roles and permissions

In this example, you will implement two user roles with specific permissions:

  • moderator: A moderator can delete all messages but not channels.
  • admin: An admin can delete all messages and channels.
supabase/migrations/init.sql
-- Custom types
create type public.app_permission as enum ('channels.delete', 'messages.delete');
create type public.app_role as enum ('admin', 'moderator');

-- USER ROLES
create table public.user_roles (
id bigint generated by default as identity primary key,
user_id uuid references auth.users on delete cascade not null,
role app_role not null,
unique (user_id, role)
);
comment on table public.user_roles is 'Application roles for each user.';

-- ROLE PERMISSIONS
create table public.role_permissions (
id bigint generated by default as identity primary key,
role app_role not null,
permission app_permission not null,
unique (role, permission)
);
comment on table public.role_permissions is 'Application permissions for each role.';

You can now manage your roles and permissions in SQL. For example, to add the mentioned roles and permissions from above, run:

supabase/seed.sql
insert into public.role_permissions (role, permission)
values
('admin', 'channels.delete'),
('admin', 'messages.delete'),
('moderator', 'messages.delete');

Create Auth Hook to apply user role

The Custom Access Token Auth Hook runs before a token is issued. You can use it to edit the JWT.

supabase/migrations/auth_hook.sql
-- Create the auth hook function
create or replace function public.custom_access_token_hook(event jsonb)
returns jsonb
language plpgsql
stable
as $$
declare
claims jsonb;
user_role public.app_role;
begin
-- Fetch the user role in the user_roles table
select role into user_role from public.user_roles where user_id = (event->>'user_id')::uuid;

claims := event->'claims';

if user_role is not null then
-- Set the claim
claims := jsonb_set(claims, '{user_role}', to_jsonb(user_role));
else
claims := jsonb_set(claims, '{user_role}', 'null');
end if;

-- Update the 'claims' object in the original event
event := jsonb_set(event, '{claims}', claims);

-- Return the modified or original event
return event;
end;
$$;

grant usage on schema public to supabase_auth_admin;

grant execute
on function public.custom_access_token_hook
to supabase_auth_admin;

revoke execute
on function public.custom_access_token_hook
from authenticated, anon, public;

grant all
on table public.user_roles
to supabase_auth_admin;

revoke all
on table public.user_roles
from authenticated, anon, public;

create policy "Allow auth admin to read user roles" ON public.user_roles
as permissive for select
to supabase_auth_admin
using (true)

Enable the hook

In the dashboard, navigate to Authentication > Hooks (Beta) and select the appropriate PostgreSQL function from the dropdown menu.

When developing locally, follow the local development instructions.

Accessing custom claims in RLS policies

To utilize Role-Based Access Control (RBAC) in Row Level Security (RLS) policies, create an authorize method that reads the user's role from their JWT and checks the role's permissions:

supabase/migrations/init.sql
create or replace function public.authorize(
requested_permission app_permission
)
returns boolean as $$
declare
bind_permissions int;
user_role public.app_role;
begin
-- Fetch user role once and store it to reduce number of calls
select (auth.jwt() ->> 'user_role')::public.app_role into user_role;

select count(*)
into bind_permissions
from public.role_permissions
where role_permissions.permission = requested_permission
and role_permissions.role = user_role;

return bind_permissions > 0;
end;
$$ language plpgsql stable security definer set search_path = '';

You can then use the authorize method within your RLS policies. For example, to enable the desired delete access, you would add the following policies:

create policy "Allow authorized delete access" on public.channels for delete to authenticated using ( (SELECT authorize('channels.delete')) );
create policy "Allow authorized delete access" on public.messages for delete to authenticated using ( (SELECT authorize('messages.delete')) );

Accessing custom claims in your application

The auth hook will only modify the access token JWT but not the auth response. Therefore, to access the custom claims in your application, e.g. your browser client, or server-side middleware, you will need to decode the access_token JWT on the auth session.

In a JavaScript client application you can for example use the jwt-decode package:

import { jwtDecode } from 'jwt-decode'

const { subscription: authListener } = supabase.auth.onAuthStateChange(async (event, session) => {
if (session) {
const jwt = jwtDecode(session.access_token)
const userRole = jwt.user_role
}
})

For server-side logic you can use packages like express-jwt, koa-jwt, PyJWT, dart_jsonwebtoken, Microsoft.AspNetCore.Authentication.JwtBearer, etc.

Conclusion

You now have a robust system in place to manage user roles and permissions within your database that automatically propagates to Supabase Auth.

More resources

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