No authentication/authorization is enforced when a server attempts to join a quorum in Apache ZooKeeper before 3.4.10, and 3.5.0-alpha through 3.5.3-beta. As a result an arbitrary end point could join the cluster and begin propagating counterfeit changes to the leader.
https://www.oracle.com/security-alerts/cpujul2020.html
https://www.debian.org/security/2018/dsa-4214