GHSA-p2j4-c4g6-rpf5

### Summary

Arc's user-SQL validator (`internal/api/query.go:ValidateSQLRequest`) blocked only `read_parquet(` and `arc_partition_agg(` via regex denylist. The broader DuckDB I/O function family — `read_csv_auto`, `read_csv`, `read_json`, `read_json_auto`, `read_text`, `read_blob`, `glob`, `parquet_metadata`, `parquet_schema`, `read_xlsx`, etc. — was not blocked. RBAC table-reference extraction inspected only `FROM`/`JOIN` clauses, so scalar table functions in the `SELECT` list slipped past both layers.

### Impact

Any authenticated user, including a token with `permissions: []`, can read arbitrary local files via:

``` POST /api/v1/query Authorization: Bearer <token> {"sql": "SELECT * FROM read_csv_auto('/etc/passwd', header=false, columns={'l':'VARCHAR'}) LIMIT 5"} ```

Confirmed reachable targets:

- `auth.db` — bcrypt hashes for every API token, plus legacy SHA-256 rows. - `arc.toml` — S3 secrets, TLS keys. - `/proc/self/environ` — environment-variable secrets. - Cross-tenant Parquet files — bypasses RBAC because the tenant scope is enforced at the table layer, not on raw file paths. - SSRF when `httpfs` is loaded (any S3-backed deployment) — `read_csv_auto('http://169.254.169.254/latest/meta-data/...')` reaches instance metadata IPs.

### Patches

Fixed in 2026.06.1 (PR #442) via a structural sandbox at the DuckDB layer:

1. `SET GLOBAL allowed_directories = [...]` enumerates Arc's legitimate filesystem prefixes (storage roots + tier prefixes + import upload dir + compaction temp). 2. `SET GLOBAL enable_external_access = false` (one-way at runtime). 3. Verified by reading back the flag.

After lockdown, DuckDB refuses to open any file outside the allowlist and refuses further `INSTALL`/`LOAD`. Already-loaded extensions remain callable.

### Workarounds

- Restrict API access to known-trusted networks via firewall rules. - Temporary mitigation: add `read_csv*`/`read_json*`/`glob` etc. to `dangerousSQLPattern` in `internal/api/query.go` pending 2026.06.1.

### Credits

Reported by Alex Manson ([@NeuroWinter](https://github.com/NeuroWinter), https://neurowinter.com/) on 2026-05-19.