CVE-2026-34478: Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility

[Piotr Karwasz <pkarwasz () apache org>]

Severity: moderate 

Affected versions:

- Apache Log4j Core (org.apache.logging.log4j:log4j-core) 2.21.0 before 2.25.4
- Apache Log4j Core (org.apache.logging.log4j:log4j-core) 3.0.0-beta1 through 3.0.0-beta3

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.

Credit:

Samuli Leinonen (finder)

References:

https://github.com/apache/logging-log4j2/pull/4074
https://logging.apache.org/security.html#CVE-2026-34478
https://logging.apache.org/cyclonedx/vdr.xml
https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-34478

Timeline:

2025-12-25: Vulnerability reported by Samuli Leinonen
2026-03-10: Candidate patch shared internally by Piotr P. Karwasz
2026-03-24: Fix shared publicly by Piotr P. Karwasz as pull request #4074
2026-03-25: Fix verified by reporter
2026-03-28: Log4j 2.25.4 released