Summary
Apache Log4j Core: log injection in Rfc5424Layout due to silent configuration incompatibility
Apache Log4j Core's 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
newLineEscapeattribute 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
useTlsMessageFormatattribute 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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-34478? CVE-2026-34478 is a medium-severity security vulnerability in org.apache.logging.log4j:log4j-core (maven), affecting versions >= 2.21.0, < 2.25.4. It is fixed in 2.25.4.
- Which versions of org.apache.logging.log4j:log4j-core are affected by CVE-2026-34478? org.apache.logging.log4j:log4j-core (maven) versions >= 2.21.0, < 2.25.4 is affected.
- Is there a fix for CVE-2026-34478? Yes. CVE-2026-34478 is fixed in 2.25.4. Upgrade to this version or later.
- Is CVE-2026-34478 exploitable, and should I be worried? Whether CVE-2026-34478 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-34478 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-34478? Upgrade
org.apache.logging.log4j:log4j-coreto 2.25.4 or later.