CVE-2026-34478 Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility
Attackers can inject CRLF sequences into logs via Rfc5424Layout due to silent configuration renames, affecting users with specific syslog setups.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34478?
CVE-2026-34478 is a medium-severity vulnerability in Apache Log4j Core, published on 10 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-34478?
4 open source packages ship an affected version, including org.apache.logging.log4j:log4j-core. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
org.apache.logging.log4j:log4j-core Maven | From 2.21.0 | 2.25.4 |
org.apache.logging.log4j:log4j-core Maven | From 2.21.0 | 2.25.4 |
org.apache.logging.log4j:log4j-core Maven | From 3.0.0-beta1 to 3.0.0-beta3 | None |
org.apache.logging.log4j:log4j-core Maven | From 3.0.0-beta1 to 3.0.0-beta3 | None |
Products include Apache Log4j.
How do I fix CVE-2026-34478?
Upgrade org.apache.logging.log4j:log4j-core to 2.25.4 or later.
Is CVE-2026-34478 being exploited?
There is no known exploitation of CVE-2026-34478 in the wild and no public exploit code.
Other vulnerabilities in Apache Log4j Core
- CVE-2021-44228 Apache Log4j2 Remote Code Execution
- CVE-2021-45046 Apache Log4j2 Deserialization of Untrusted Data
- CVE-2022-23305 SQL injection in JDBC Appender in Apache Log4j V1
- CVE-2017-5645 Deserialization of Untrusted Data in Log4j
- CVE-2022-23302 Deserialization of untrusted data in JMSSink in Apache Log4j 1.x
Sources
Also known as CLEANSTART-2026-CP64029, CLEANSTART-2026-DZ37385, CLEANSTART-2026-GW86512, CLEANSTART-2026-ZG68891, CLEANSTART-2026-ZR66277, ECHO-d811-dae2-2041 and 69 more.