The vulnerability stems from improper security detection in ZKUtil.isSecureZooKeeper() which controls ZooKeeper ACL assignments. The patched commits show critical modifications to the security validation logic:

1. Expanded JAAS configuration checks from just 'Client' to include Kerberos-specific entries
2. Removed the keytab file presence requirement from the security check These changes indicate the original function was failing to properly detect secure configurations, leading to weak ACLs. During exploitation, this function would be actively involved in security evaluation, making it appear in stack traces when ZooKeeper ACLs are being set. The function's incorrect security assessment directly enabled the improper access control described in CVE-2015-1836.