Description Nimbus instantiates the class named by a topology's "topology.scheduler.strategy" when scheduling it. The allowlist intended to constrain that choice, "nimbus.scheduler.strategy.class.whitelist", was treated as "allow any class" when unset, and it had no entry in the shipped "defaults.yaml", so unset was the shipped state. An authenticated submitter could therefore cause Nimbus to instantiate any class on the Nimbus classpath that has a no-argument constructor. Storm's own production artifacts contain classes with side effects suitable for this: instantiating "org.apache.storm.testing.InProcessZookeeper" starts a ZooKeeper server with its own threads and sockets and nothing closes it, and "org.apache.storm.LocalCluster" behaves similarly. Repeated submissions therefore consume threads, sockets and memory in the most privileged daemon in the cluster. Mitigation Upgrade to 3.1.0, where an unset allowlist means the strategies shipped with Storm rather than any class. Note that this is a behaviour change: a cluster running an out-of-tree scheduler strategy without an explicit allowlist will have those topologies refused until the class is added to "nimbus.scheduler.strategy.class.whitelist". The rejection message names the refused class and the configuration key. Users who cannot upgrade immediately should set "nimbus.scheduler.strategy.class.whitelist" explicitly to the strategies they intend to permit.