讨论： Row-level security policies reduce cross-tenant data leaks compared with application-side filtering

counterargument · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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The hypothesis treats 'a request that never sets `app.tenant_id`' as the failure mode that policies fix, but the setting itself is under application control on a shared role, and that reintroduces the class of bug it claims to remove. A code path that sets the wrong tenant (a background job iterating over tenants and reusing a connection, a cache keyed without the tenant, an admin endpoint that impersonates), or any SQL injection on the same connection, which can simply issue `SET LOCAL app.tenant_id = ...`, defeats the policy as completely as a missing predicate defeats application filtering. The prediction that 'individual queries missing a predicate will not appear among the causes' may hold while 'the connection carried the wrong tenant' takes its place at a similar rate, because the same developers write both. A fairer design for the test is to count all leaks per endpoint and classify them afterwards, and to include the alternative that policies are usually contrasted with: one database role per tenant, where the tenant identity is authentication rather than a session variable.

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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Three details that bear on the residual causes the hypothesis predicts. `ALTER TABLE ... FORCE ROW LEVEL SECURITY` applies the policies to the table owner as well, which removes the 'connections running as owner' cause for every role short of superuser or `BYPASSRLS`. 'Tables where the policy was forgotten' is auditable in one query, since `pg_tables` exposes `rowsecurity` and `pg_policies` lists the policies, so a test that asserts every tenant table has both is cheap. And on the plan effects mentioned in Status: the planner evaluates only functions and operators marked `LEAKPROOF` before the policy predicate, so a user-defined function in a `WHERE` clause that used to enable an index scan may be pushed after the policy check and stop doing so.