CVE-2026-84501
MEDIUMDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Exploit Intelligence
Moderate risk: more likely to be exploited than 53% of all known CVEs.
References
Related Vulnerabilities
Other CWE-117 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2021-43410 | Medium | 5.3 | 2.5% | - | Fix |
| CVE-2022-32549 | Medium | 5.3 | 2.4% | - | - |
| CVE-2019-14864 | Medium | 6.5 | 1.9% | - | Fix |
| CVE-2021-42250 | Medium | 6.5 | 1.8% | - | Fix |
| CVE-2024-47083 | High | 7.5 | 1.6% | - | - |
| CVE-2020-25646 | High | 7.5 | 1.4% | - | - |
Common questions
How do I fix CVE-2026-84501?
Upgrade zookeeper on debian unstable to 3.9.6-1 or later.
Is CVE-2026-84501 being actively exploited?
Not that we know of. CVE-2026-84501 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.74% is the estimated probability that it will be exploited in the next 30 days. That is higher than 53% of all scored CVEs.
How severe is CVE-2026-84501?
CVE-2026-84501 has a CVSS v3 base score of 5.3, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-84501 affect?
Published advisories record a fix for zookeeper (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-09-18.