CVE-2026-34478

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

State: Published
Severity: 6.9 Medium
Published: Apr 10, 2026
Updated: Apr 10, 2026
Reserved: Mar 28, 2026

Description

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 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.

Scores

CVSS	Score	Vector	Source
4.0	6.9 Medium	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	apache

Weaknesses

CWE-684 Incorrect Provision of Specified Functionality
CWE-117 Improper Output Neutralization for Logs

Affected Products

Product	Versions
Apache Log4j CoreApache Software Foundation	2.21.0 to before 2.25.4Affected 3.0.0-beta1 through 3.0.0-beta3Affected

References

https://github.com/apache/logging-log4j2/pull/4074patch
https://logging.apache.org/security.html#CVE-2026-34478vendor-advisory
https://logging.apache.org/cyclonedx/vdr.xmlvendor-advisory
https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layoutrelated
https://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwtvendor-advisory
http://www.openwall.com/lists/oss-security/2026/04/10/7

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