Security(M-03): missing role-filter state falls back to mounting every group in the token

[dcmcand]

Summary

When role-based mount filtering is unavailable, the shared-storage resolver falls back to the user's full groups claim. The static allowlist is empty by default, so every group the user holds becomes a writable shared mount. This turns ordinary realm group membership into shared-storage authorization on any deployment where the RBAC bootstrap is off or partially configured.

Severity: Medium · CWE-863 (Incorrect Authorization)
Validation: Confirmed against HEAD f932d80 on 2026-07-14 (assessed at 69c84f7; unchanged since).

Evidence

• config/jupyterhub/01-spawner.py:794-806: uses groups_with_permission_to_mount when the key is present, otherwise raw_groups = auth_state.get("groups", []) (the full claim).
• config/jupyterhub/01-spawner.py:810-812: the allowlist only filters when it is non-empty; an empty list is falsy and skips filtering entirely.
• Default allowlist is empty: 01-spawner.py:147 reads custom.shared-storage-groups defaulting to [], and values.yaml:471 sets shared-storage-groups: [].
• Documented as legacy: values.yaml:491-495 states that with RBAC off the spawner "falls back to mounting every group the user is in (legacy behaviour)."
• Corroboration: 00-gateway-auth.py:434 returns early without setting groups_with_permission_to_mount when realm_api_url is unset, so the key is genuinely absent when RBAC is off.

Impact

In nonstandard, disabled-bootstrap, or partially configured deployments, any realm group a user belongs to becomes a writable shared mount, with no explicit approval step.

Remediation

• Make the authorization mode explicit.
• In production, treat missing role-filter state as zero mounts.
• Require a non-empty approved mapping.
• Put the legacy full-groups behavior behind an explicit unsafe-dev flag.

Acceptance criteria

• Absent, malformed, or unavailable role-filter data yields no shared mounts and a visible readiness/status error.

_{Source: data-science-pack 0.1.0 security assessment (pinned commit 69c84f72df259ec755ed40bfc83f20158c550d55), finding M-03.}