CVE-2026-84501: Apache ZooKeeper: Operational log forgery via newline injection in EnsembleAuthenticationProvider

[Andor Molnar <an...@apache.org>]

Severity: moderate 

Affected versions:

- Apache ZooKeeper (org.apache.zookeeper:zookeeper) 3.9.0 through 3.9.5
- Apache ZooKeeper (org.apache.zookeeper:zookeeper) 3.8.0 through 3.8.6

Description:

An unauthenticated attacker can inject arbitrary fake log lines into Apache ZooKeeper's operational log by sending a crafted add_auth("ensemble", ...) request containing newline characters (\n). When the ensemble name doesn't match, EnsembleAuthenticationProvider.handleAuthentication() logs the raw, unsanitized name via LOG.warn(). Because SLF4J's {} placeholder preserves embedded newlines, the attacker can forge complete log entries — with arbitrary timestamps, log levels, class names, and messages — that are visually indistinguishable from genuine ZooKeeper log output.

This issue affects Apache ZooKeeper: from 3.9.0 through 3.9.5, from 3.8.0 through 3.8.6.

Users are recommended to upgrade to version 3.8.7 or 3.9.6, which fixes the issue.

Credit:

Youlong Chen Institute of Computing Technology <ch...@ict.ac.cn> (finder)

References:

https://zookeeper.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-84501