CVE-2026-24281: Apache ZooKeeper: Reverse-DNS fallback enables hostname verification bypass in ZooKeeper ZKTrustManager

[Andor Molnar <andor () apache org>]

Severity: important 

Affected versions:

- Apache ZooKeeper (org.apache.zookeeper:zookeeper) 3.9.0 through 3.9.4
- Apache ZooKeeper (org.apache.zookeeper:zookeeper) 3.8.0 through 3.8.5

Description:

Hostname verification in Apache ZooKeeper ZKTrustManager falls back to reverse DNS (PTR) when IP SAN validation fails, 
allowing attackers who control or spoof PTR records to impersonate ZooKeeper servers or clients with a valid 
certificate for the PTR name. It's important to note that attacker must present a certificate which is trusted by 
ZKTrustManager which makes the attack vector harder to exploit. Users are recommended to upgrade to version 3.8.6 or 
3.9.5, which fixes this issue by introducing a new configuration option to disable reverse DNS lookup in client and 
quorum protocols.

This issue is being tracked as ZOOKEEPER-4986 

Credit:

Nikita Markevich <markevich.nikita1 () gmail com> (reporter)

References:

https://zookeeper.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-24281
https://issues.apache.org/jira/browse/ZOOKEEPER-4986