The _on_reaction approval handler used:
if self._allowed_user_ids and sender not in self._allowed_user_ids:
When MATRIX_ALLOWED_USERS is not configured, _allowed_user_ids is an
empty set. The short-circuit on the empty set caused the deny block to
never execute, allowing any Matrix room member to approve or deny tool
calls via ✅/❎ reactions — even users that run.py's _is_user_authorized
would reject for regular messages.
Fix mirrors the Telegram _is_callback_user_authorized fix (commit
89d32052e, PR #28494): deny by default when no allowlist is configured,
unless GATEWAY_ALLOW_ALL_USERS=true is explicitly set.
784d8dd2c2