CVE-2026-34478
Apache Log4j Core: log injection in `Rfc5424Layout` due to silent configuration incompatibility
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.
How to fix CVE-2026-34478
To remediate CVE-2026-34478, upgrade the affected package to a fixed version below.
- —upgrade to 2.25.4 or later
Is CVE-2026-34478 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- >= 2.21.0, < 2.25.4
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | 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 |