CVE-2026-84501: An unauthenticated attacker can inject arbitrary fake log lines into Apache ZooKeeper's operational log by sen

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MEDIUM 5.3REMOTEWATCHLISTUPGRADE

CVE-2026-84501 is a medium-severity flaw (CVSS 5.3) affecting zookeeper. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 arbit

Why this priority

Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 46%)
CISA exploited list	No
Weakness	CWE-117
Published	2026-09-16
Last modified	2026-09-18

Publisher (CNA): Apache Software Foundation · Vendor: apache

Last updated 2026-09-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-16 10:16 UTCPublished CVE record published
2026-09-18 14:36 UTCUpdated Record last modified

apache zookeeper UPGRADE

Affected: >= 3.8.0 and < 3.8.7 · >= 3.9.0 and < 3.9.6

Fixed in: 3.8.7, 3.9.6

Safe to downgrade to: a release before 3.8.0

1. Check installed version

apt-cache policy zookeeper || rpm -q zookeeper || zookeeper --version

2. UPGRADE to 3.8.7 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade zookeeper

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade zookeeper

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.8.0 (Debian / Ubuntu)

apt-cache madison zookeeper sudo apt install zookeeper=<VERSION_BEFORE_3.8.0> sudo apt-mark hold zookeeper

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list zookeeper sudo dnf downgrade zookeeper-<VERSION_BEFORE_3.8.0> sudo dnf versionlock add zookeeper

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-84501?

zookeeper: upgrade to 3.8.7, 3.9.6. Exact commands for each OS are on this page.

Is CVE-2026-84501 being exploited?

CVE-2026-84501 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-84501 affect?

CVE-2026-84501 affects zookeeper. Severity is medium (CVSS 5.3). 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, Ensembl

How urgent is CVE-2026-84501?

Priority 42/100. Remotely exploitable, no login, no user action; On your watchlist

Software pages

Apache ZooKeeper vulnerabilities

Related CVEs

CVE-2026-84439 MEDIUM 5.3 UPGRADE When audit logging is enabled (zookeeper.audit.enable=true), an unauthenticated attacker can inject arbitrary
CVE-2026-79993 HIGH 7.5 UPGRADE The `deleteContainer` opcode (0x14/20) is processed without verifying the caller's ACL permissions, allowing a
CVE-2026-59739 HIGH 7.5 UPGRADE Information disclosure via SetWatches reconnect replay in Apache ZooKeeper due to missing ACL check. An attack
CVE-2026-59969 HIGH 7.5 UPGRADE Apache ZooKeeper quorum TLS fails to enforce peer hostname verification in FIPS-mode deployments

References

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