# CVE-2026-84501: zookeeper: An unauthenticated attacker can inject arbitrary fake log lines into Apache ZooKeeper's operational log by sending a...

CVE-2026-84501 is fixed in zookeeper on 1 platform by DEBIAN-CVE-2026-84501 (version 3.9.6-1), released September 26, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-84501
- Priority: Routine
- CVSS: 5.3 (medium)
- EPSS: 0.0074
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 16, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | zookeeper Debian 14 | 3.9.6-1 | [DEBIAN-CVE-2026-84501](https://patchblox.com/security/data/fix/linux/debian-cve-2026-84501.md) | Sep 26, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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