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.