CVE-2026-34478: Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in

This month

HIGH 7.5REMOTEWATCHLISTUPGRADE

CVE-2026-34478 is a high-severity flaw (CVSS 7.5) affecting log4j. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0 through 2.25.3, is vulnerable to log injection via CRLF sequences due to undocumented renames of security-relevant configuration attributes. Two distinct issues affect users of stream-based syslog services who configure Rfc5424Layout directly: * The newLineEscape attribute was silently renamed, causing newline escapi

Why this priority

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

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.0% (top 38%)
CISA exploited list	No
Weakness	CWE-117, CWE-684
Published	2026-04-10
Last modified	2026-10-26

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

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-10 16:16 UTCPublished CVE record published
2026-10-26 22:10 UTCUpdated Record last modified

apache log4j UPGRADE

Affected: >= 2.21.0 and < 2.25.4 · 3.0.0 beta1 · 3.0.0 beta2 · 3.0.0 beta3

Fixed in: 2.25.4

Safe to downgrade to: a release before 2.21.0

1. Check installed version

apt-cache policy log4j || rpm -q log4j || log4j --version

2. UPGRADE to 2.25.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade log4j

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade log4j

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 2.21.0 (Debian / Ubuntu)

apt-cache madison log4j sudo apt install log4j=<VERSION_BEFORE_2.21.0> sudo apt-mark hold log4j

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

dnf --showduplicates list log4j sudo dnf downgrade log4j-<VERSION_BEFORE_2.21.0> sudo dnf versionlock add log4j

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-2026-34478?

log4j: upgrade to 2.25.4. Exact commands for each OS are on this page.

Is CVE-2026-34478 being exploited?

CVE-2026-34478 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-2026-34478 affect?

CVE-2026-34478 affects log4j. Severity is high (CVSS 7.5). Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0 through 2.25.3, is vulnerable to log injection via CRLF sequences due to undocumented renames of security-relevan

How urgent is CVE-2026-34478?

Priority 52/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Apache Log4j vulnerabilities

Related CVEs

CVE-2026-34480 HIGH 7.5 UPGRADE Apache Log4j Core's XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in version
CVE-2026-34481 HIGH 7.5 UPGRADE Apache Log4j's JsonTemplateLayout https://logging.apache.org/log4j/2.x/manual/json-template-layout.html , in
CVE-2026-34477 MEDIUM 5.9 UPGRADE The fix for CVE-2025-68161 https://logging.apache.org/security.html#CVE-2025-68161 was incomplete: it addres
CVE-2026-34479 HIGH 7.5 UPGRADE The Log4j1XmlLayout from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML
CVE-2026-49844 MEDIUM 5.9 UPGRADE Improper encoding of non-finite floating-point values during MapMessage JSON serialization in Apache Log4j API

References

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