Supabase Row Level Security (RLS) is PostgreSQL's built-in access control system. When it's set up correctly, your database enforces authorization at the data layer — not just in your API. When it's set up wrong, your users see each other's data and you don't find out until someone reports a bug. I've shipped three production apps with Supabase RLS. Here's what I've learned. Without RLS, any query to your Supabase table returns all rows. With RLS enabled and policies defined, PostgreSQL filters rows automatically based on the current user context. -- Enable RLS on a table ALTER TABLE posts ENABLE ROW LEVEL SECURITY; -- Now: zero rows returned for everyone until you add policies -- SELECT * FROM posts; → [] The current user context in Supabase is the JWT from your auth session. Supabase exposes it via auth.uid() inside your policies. For any table owned by users, you want four policies — one per operation: -- Users can only read their own posts CREATE POLICY "users_read_own_posts" ON posts FOR SELECT USING (auth.uid() = user_id); -- Users can only insert rows where they're the owner CREATE POLICY "users_insert_own_posts" ON posts FOR INSERT WITH CHECK (auth.uid() = user_id); -- Users can only update their own posts CREATE POLICY "users_update_own_posts" ON posts FOR UPDATE USING (auth.uid() = user_id) WITH CHECK (auth.uid() = user_id); -- Users can only delete their own posts CREATE POLICY "users_delete_own_posts" ON posts FOR DELETE USING (auth.uid() = user_id); The difference between USING and WITH CHECK: USING — filters which existing rows are visible/modifiable WITH CHECK — validates the row being written For INSERT, only WITH CHECK applies (there's no existing row). For DELETE, only USING applies. For UPDATE, use both. This is the most dangerous misunderstanding: // This bypasses ALL RLS policies — use for admin operations only const adminClient = createClient(url, process.env.SUPABASE_SERVICE_ROLE_KEY!); // This respects RLS — use for user-facing operations const userClient = createClient(url, process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!); If you're using the service role key in your frontend or in API routes that handle user requests, you've disabled RLS for those routes. I've seen codebases where the developer used the service role key everywhere "to avoid RLS complexity" — and their users could read each other's data. Rule: service role key only in server-side admin code. Anon key for everything users touch. -- team_members junction table CREATE TABLE team_members ( team_id uuid REFERENCES teams(id), user_id uuid REFERENCES auth.users(id), role text NOT NULL ); -- Wrong: only adding policy to teams table -- The team_members table also needs policies -- Correct: policy on the junction table CREATE POLICY "team_members_read" ON team_members FOR SELECT USING ( auth.uid() = user_id -- You can see your own membership OR EXISTS ( SELECT 1 FROM team_members tm WHERE tm.team_id = team_members.team_id AND tm.user_id = auth.uid() ) -- You can see other members of teams you're in ); Every table with sensitive data needs its own RLS policy. Forgetting a junction table is a common leak. This is the junction table pattern above done wrong: -- BROKEN: this policy on team_members queries team_members itself CREATE POLICY "team_members_read" ON team_members FOR SELECT USING ( EXISTS ( SELECT 1 FROM team_members -- Recursive! WHERE team_id = team_members.team_id AND user_id = auth.uid() ) ); Postgres will detect the recursion and throw an error, but it's a runtime error that surfaces as a 500 in production. Fix: use a security definer function to break the cycle: -- Function runs as the table owner, bypasses RLS for the check CREATE OR REPLACE FUNCTION user_is_team_member(check_team_id uuid) RETURNS boolean LANGUAGE sql SECURITY DEFINER AS $$ SELECT EXISTS ( SELECT 1 FROM team_members WHERE team_id = check_team_id AND user_id = auth.uid() ); $$; -- Policy using the function CREATE POLICY "team_members_read" ON team_members FOR SELECT USING (user_is_team_member(team_id)); Supabase's default grants give the anon role SELECT access to your tables. If you enable RLS with only an authenticated-user policy, unauthenticated requests return empty results — but they don't error. That's usually fine. The problem: if you write a policy that doesn't check auth.uid() (e.g., "public posts are readable by anyone"), you may inadvertently make those rows readable to unauthenticated users too. Be explicit: -- Explicitly allow public read for published posts CREATE POLICY "published_posts_public_read" ON posts FOR SELECT USING (published = true); -- Anyone (including anon) can read published posts -- Only authenticated users can read drafts CREATE POLICY "draft_posts_auth_read" ON posts FOR SELECT USING ( published = false AND auth.uid() = user_id ); The pattern I use for SaaS with teams/organizations: -- Check if current user belongs to an org CREATE OR REPLACE FUNCTION user_org_ids() RETURNS uuid[] LANGUAGE sql SECURITY DEFINER STABLE AS $$ SELECT array_agg(organization_id) FROM organization_members WHERE user_id = auth.uid(); $$; -- Resources are visible to all org members CREATE POLICY "org_resources_read" ON resources FOR SELECT USING (organization_id = ANY(user_org_ids())); CREATE POLICY "org_resources_insert" ON resources FOR INSERT WITH CHECK (organization_id = ANY(user_org_ids())); The STABLE annotation on the function lets Postgres cache the result within a single query, so querying a list of resources doesn't execute the membership check N times. Don't trust policies until you've tested them as a real user: -- Impersonate a user in the SQL editor SET LOCAL request.jwt.claim.sub = 'user-uuid-here'; SET LOCAL role = 'authenticated'; -- Now your queries run as that user SELECT * FROM posts; -- Should only return that user's posts Or write automated tests: // Create two test users const user1 = await supabase.auth.signUp({ email: '[email protected]', password: 'test' }); const user2 = await supabase.auth.signUp({ email: '[email protected]', password: 'test' }); // User 1 creates a post const client1 = createClient(url, anonKey, { auth: { persistSession: false } }); await client1.auth.signInWithPassword({ email: '[email protected]', password: 'test' }); const { data: post } = await client1.from('posts').insert({ title: 'Test' }).select().single(); // User 2 should NOT see it const client2 = createClient(url, anonKey, { auth: { persistSession: false } }); await client2.auth.signInWithPassword({ email: '[email protected]', password: 'test' }); const { data: posts } = await client2.from('posts').select(); assert(posts?.find(p => p.id === post.id) === undefined); // Must be empty RLS policies run on every query. Complex policies with subqueries can hurt performance. Mitigations: Index the columns your policies filter on (user_id, organization_id, etc.) Use SECURITY DEFINER functions to avoid repeated subqueries Check EXPLAIN ANALYZE on your most frequent queries with RLS enabled In practice, well-written RLS policies on indexed columns add <5ms overhead. Unindexed subquery policies on large tables can add hundreds of milliseconds. Think of RLS policies as WHERE clauses that PostgreSQL appends to every query automatically. USING (user_id = auth.uid()) becomes WHERE user_id = auth.uid() on every SELECT. Once you see it that way, writing correct policies becomes intuitive. The starter kit I ship has Supabase auth, RLS policies for user-owned resources, the org/team multi-tenancy pattern, and service-role separation all configured from day one: AI SaaS Starter Kit ($99) — Next.js 15 + Supabase + Drizzle + Stripe + Claude API. Skip the security setup and ship. 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