Limit grant-uploads storage access to each file's uploader

[SpikeyCoder]

Problem

grant-uploads is a private bucket, but its storage.objects policies (created outside migrations) only checked bucket_id:

Policy	Role	Check
Allow anon uploads / reads / deletes	anon	bucket_id = 'grant-uploads'
Allow authenticated uploads / reads / deletes	authenticated	bucket_id = 'grant-uploads'

The anon key ships in the frontend, so anyone could list, download and delete every user's uploaded past grants (AI Grant Writer) and reference documents (Project Workspace), signed in or not, and upload files into the bucket. I confirmed against live pg_policies that these six are the bucket's only policies and that each one covers only grant-uploads.

Fix

• supabase/migrations/20261013030000_grant_uploads_owner_policies.sql drops those six policies and adds three for authenticated only:

  • INSERT: a user can upload only files recorded as theirs.
  • SELECT: a user can list and read only their own files.
  • DELETE: a user can delete only their own files.

  Each one requires owner_id = auth.uid()::text. Anon gets no access. There's no UPDATE policy: nothing overwrites an upload (upsert: false). Ownership is checked by owner_id, which storage sets from the uploader's JWT and grant-writer already checks, not by path. The Grant Writer's paths (<session>/<uuid>.<ext>) carry no user id. Both existing objects have owner_id set.

• supabase/functions/ai-draft/index.ts: ai-draft downloads a knowledge-base row's storage_path with the service role. storage_path is user-writable, so a user could point their own row at another user's file and get its text into their draft.

  A path-prefix check alone isn't enough. storage-js puts the path into the request URL unencoded, so reference-docs/<me>/%2e%2e/%2e%2e/<victim>/<file> resolves as ... ai-draft now reads a file only when all of these hold:

  • its path has Project Workspace's upload shape, reference-docs/<uuid>/<uuid>/<digits>.<ext>, where <ext> has no dot, slash, backslash, %, ?, # or control character, so nothing can resolve elsewhere;
  • the path is in the caller's own folder;
  • storage.objects records the caller as owner_id, the same check grant-writer does.

  A failed ownership lookup is logged, and no file is read.

• src/pages/GrantWriter.tsx: signed-out visitors see a "Sign in to upload past grants" link in place of the past-grants drop zone. It appears only once auth has loaded, so signed-in users never see it while their session restores. grant-writer needs a signed-in user anyway, so their uploads were never usable. The upload function keeps a sign-in check as a backstop.

Testing

Ran against a local Postgres 16 with storage.objects, auth.uid(), the anon/authenticated roles and the current production policies recreated.

Check	Before	After
anon lists grant-uploads	sees all 2	sees 0
anon uploads	allowed	RLS error
anon deletes	deletes all	deletes 0
anon uploads to bug-screenshots (bug-reports/)	allowed	allowed (unchanged)
user A lists	sees A's and B's	sees only A's
user A uploads own file	allowed	allowed
user A uploads a file with B's owner_id, or no owner	allowed	RLS error
user A deletes B's file	deletes it	deletes 0
user A deletes own file	allowed	allowed

The ai-draft path rule accepts real upload paths, including names without a dot such as 1728.Final Proposal. It rejects %2e%2e, .., extra segments, backslashes, ?, # and newlines.

The migration also applies cleanly twice in a row. tsc -b, eslint on GrantWriter.tsx and npm test (70/70) pass. deno check on ai-draft reports the same 2 errors as main, both pre-existing.

/code-review and /security-review were run, then run again on the fixes until both came back with nothing left to fix.

Deploy

1. Apply the migration.
2. Deploy ai-draft.

The frontend ships with the merge.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UPYZwv6X3kmDoJN7g4RaJm

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