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.
CWE-862 - Missing Authorization
The missing authorization vulnerability occurs when a software program allows users to access privileged parts of the program without verifying the user credentials. Impact of such a vulnerability depends on the resources employed by the software, ranging from account takeover to sensitive information exposure, denial of service, and complete system takeover.