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

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-16 · updated 2026-09-18
Weakness: CWE-117

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.

Affected products

Apache Software Foundation Apache ZooKeeper: from 3.8.0 before 3.8.7; from 3.9.0 before 3.9.6 — fixed in 3.8.7, 3.9.6

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache ZooKeeper 3.8.7, 3.9.6. Check the vendor advisory for mitigations.

References

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