Medium6.9Apache Software Foundation

Medium [CVE-2026-34478] Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0…

This medium-severity Apache Software Foundation advisory covers CVE-2026-34478 affecting Apache Log4j, Apache Log4j Core, Log4j.

CVE-2026-34478 Published Apr 10, 2026Updated by vendor Jun 17, 2026

CVE-2026-34478

Affected products & platforms

Apache Software FoundationLog4j

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Summary

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.

Affected versions

through 2.25.3

Official advisory · high-confidence parse· fetched 2 months ago·verify at source

Fixed versions

2.25.4

Official advisory · high-confidence parse· fetched 2 months ago·verify at source

Mitigation

Upgrade to a fixed release: 2.25.4. That is the remediation for this advisory.

The vendor advisory may list additional interim mitigations or workarounds not captured here — review it before change work.

Official advisory · high-confidence parse· fetched 2 months ago·verify at source

Sources

Official and corroborating records used to build this advisory.

NVD discovery recordNVD
Independent discovery and scoring cross-reference. Vendor remediation guidance takes precedence. Discovered from the machine-readable feed https://services.nvd.nist.gov/rest/json/cves/2.0; this link opens the NVD record for CVE-2026-34478 instead of the raw feed document.
NIST National Vulnerability Database

Cross-references

Validate CVE metadata against the authoritative registries.

CVE-2026-34478NVD CVE.org MITRE

Official resources

ASF Security — reporting & CVE process
Official documentation
The Apache Software Foundation's official security page: how ASF CVEs are published and where each project's advisories live.
Apache project downloads (fixed releases)
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