Apache Log4j Core: log injection in Rfc5424Layout due to silent configuration incompatibility

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N · score source

EPSS

• CVE-2026-34478: 0.97% estimated exploitation activity in the next 30 days; percentile 62.02 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Apache Log4j Core's 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 escaping to stop working for users of TCP framing (RFC 6587), exposing them to CRLF injection in log output. The useTlsMessageFormat attribute was silently renamed, causing users of TLS framing (RFC 5425) to be silently downgraded to unframed TCP (RFC 6587), without newline escaping. Users of the SyslogAppender are not affected, as its configuration attributes were not modified. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue.

Published by the source

10 Apr 2026, 18:31 UTC

Last modified by the source

14 Apr 2026, 00:13 UTC

Source last checked

26 Oct 2026, 16:45 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-445c-vh5m-36rj

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-34478

Advisory: Apache Log4j Core: log injection in `Rfc5424Layout` due to silent configuration incompatibility
IDs: CVE-2026-34478, GHSA-445c-vh5m-36rj
Record: https://patchnext.com/advisories/cve-2026-34478 (revision 1, source checked 26 Oct 2026, 16:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: org.apache.logging.log4j:log4j-core 2.25.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.apache.logging.log4j:log4j-core: affected >= 2.21.0, < 2.25.4; >= 3.0.0-beta1, <= 3.0.0-beta3; fixed in 2.25.4

## Suggested inspection (read-only)
- Maven org.apache.logging.log4j:log4j-core: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.apache.logging.log4j:log4j-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-445c-vh5m-36rj (GitHub-reviewed advisory GHSA-445c-vh5m-36rj lists affected ranges: Maven org.apache.logging.log4j:log4j-core >=2.21.0 =3.0.0-beta1 <=3.0.0-beta3)

Commands are suggestions: review each one before running it.