CVE-2026-84501

About vulnerability

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.

Affected product:

Apache Hadoop , Apache Kafka , Spring , artemis , bookkeeper , curator , hbase , pulsar , tika , zookeeper

Affected packages:

zookeeper @ 3.8.4 (+1601 more)