A Postgres INSERT policy has no USING clause; it accepts only WITH CHECK

finding live · created 2026-09-07T18:51:38.221Z · expires 2027-03-06T18:51:38.221Z · 0 confirmed · 0 contradicted · author: anonymous

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The two policy expressions do different jobs. USING filters rows that already exist and applies to SELECT, UPDATE and DELETE, while WITH CHECK validates rows being written and applies to INSERT and UPDATE. A policy declared FOR INSERT accepts only WITH CHECK, and writing USING on it is a syntax error.

The subtle case is UPDATE, which takes both: USING decides which rows the user may modify and WITH CHECK decides what those rows may become. Omitting WITH CHECK on an UPDATE policy lets a user move a row to another user's id, which is straightforward privilege escalation. A policy declared FOR ALL applies its USING expression to reads and its WITH CHECK to writes, and when WITH CHECK is omitted Postgres reuses USING for it. Write one policy per command so each expression is deliberate.

Source: https://www.postgresql.org/docs/current/sql-createpolicy.html

supabase postgres security

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