CVE-2026-84501: zookeeper — Fix

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.

Is your site exposed to CVE-2026-84501?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

5.3

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS — Exploit Prediction

0.0074

Probability of exploitation

0.54%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-117 CWE-117

Affected Products

Vendor	Product	Versions
apache	zookeeper	3.8.0 — 3.8.7
apache	zookeeper	3.9.0 — 3.9.6

References

Advisories & Patches

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

Other References

http://www.openwall.com/lists/oss-security/2026/09/15/7

Frequently Asked Questions

What is CVE-2026-84501? +

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. It has a CVSS v3.1 base score of 5.3 (MEDIUM).

How severe is CVE-2026-84501? +

CVE-2026-84501 has a CVSS v3.1 score of 5.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0074, placing it in the 1th percentile for exploitation probability.

What products are affected by CVE-2026-84501? +

CVE-2026-84501 affects products from apache, specifically: zookeeper. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-84501? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-125122

compose-lint is a Docker Compose security linter that identifies dangerous misconfigurations before deployment. In version 0.31.0, compose-lint does not sanitize …

CVE-2026-125121

compose-lint is a Docker Compose security linter that identifies dangerous misconfigurations before deployment. In version 0.31.0, compose-lint escapes GitHub Actions …

CVE-2026-10745

Improper output neutralization for logs vulnerability in upKeeper Solutions upKeeper Instant Privilege Access on Windows allows Log Injection-Tampering-Forging. This issue …

CVE-2026-125125

compose-lint is a Docker Compose security linter that identifies dangerous misconfigurations before deployment. From version 0.30.0 until 0.32.0, the compose-lint …

CVE-2026-125123

compose-lint is a Docker Compose security linter that identifies dangerous misconfigurations before deployment. In version 0.31.0, compose-lint's argument parser writes …

CVE-2026-118994

CallBackery versions before 0.62.0 for Perl allow log injection via newlines in the login name. login() writes the login name …