Description `SimpleACLAuthorizer` evaluated the user-level command set by returning early when `nimbus.users` was empty, before `nimbus.gro

Description

SimpleACLAuthorizer evaluated the user-level command set by returning early when nimbus.users was empty,
before nimbus.groups was considered. An operator who restricted cluster access by group alone, leaving
nimbus.users unset, therefore received no restriction at all: every authenticated principal was permitted
every user-level operation, including submitTopology, beginFileUpload and getNimbusConf.

docs/SECURITY.md presents nimbus.groups as a supported way to lock down a cluster, so a deployment
following the documentation could believe it was restricted while it was not. The failure is silent; nothing
in the logs or the configuration indicates that the group list is being ignored.

Both lists left empty continues to mean that no restriction is configured, which is the shipped default and
is unchanged.

Mitigation

Upgrade to 3.1.0, where nimbus.groups is evaluated whether or not nimbus.users is set.

Users who cannot upgrade immediately should additionally populate nimbus.users with the intended
principals, since a non-empty user list causes the group list to be evaluated on affected versions.
Operators should review Nimbus access logs for operations by principals outside the intended groups.

Note that after upgrading, a cluster configured with nimbus.groups alone becomes restrictive for the first
time. This includes NimbusClient, which calls getLeader on every connection, so clients outside the
configured groups will begin to be refused.

Credit

The ASF -- found using Claude agents to study the security of open-source projects, validated and reported by Apache Storm.