CVE-2026-84501: Apache Zookeeper

Medium severity, CVSS 5.3. EPSS: 0.7% chance of exploitation in the next 30 days.

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 products

• Apache Zookeeper: from 3.8.0, before 3.8.7 (fixed in 3.8.7); from 3.9.0, before 3.9.6 (fixed in 3.9.6)

Published 2026-09-16. Last modified 2026-09-18.