ZooKeeper Log Injection via add_auth (3.8.03.8.6, 3.9.03.9.5)
CVE-2026-84501 Published on September 16, 2026
Apache ZooKeeper: Operational log forgery via newline injection in EnsembleAuthenticationProvider
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.
Vulnerability Analysis
CVE-2026-84501 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, with no impact on integrity, and no impact on availability.
Weakness Type
Improper Output Neutralization for Logs
The software does not neutralize or incorrectly neutralizes output that is written to logs.
Products Associated with CVE-2026-84501
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Affected Versions
Apache Software Foundation Apache ZooKeeper:- Version 3.9.0, <= 3.9.5 is affected.
- Version 3.8.0, <= 3.8.6 is affected.