Summary
Apache Log4j 1 to Log4j 2 bridge: silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters
The Log4j1XmlLayout from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML 1.0 standard, producing malformed XML output. Conforming XML parsers are required to reject documents containing such characters with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records.
Two groups of users are affected:
- Those using
Log4j1XmlLayoutdirectly in a Log4j Core 2 configuration file. - Those using the Log4j 1 configuration compatibility layer with
org.apache.log4j.xml.XMLLayoutspecified as the layout class.
Users are advised to upgrade to Apache Log4j 1-to-Log4j 2 bridge version 2.25.4, which corrects this issue.
[!NOTE]
The Apache Log4j 1-to-Log4j 2 bridge is deprecated and will not be present in Log4j 3. Users are encouraged to consult the
Log4j 1 to Log4j 2 migration guide, and specifically the section on eliminating reliance on the bridge.
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-2026-34479? CVE-2026-34479 is a medium-severity security vulnerability in org.apache.logging.log4j:log4j-1.2-api (maven), affecting versions >= 2.7, < 2.25.4. It is fixed in 2.25.4.
- Which versions of org.apache.logging.log4j:log4j-1.2-api are affected by CVE-2026-34479? org.apache.logging.log4j:log4j-1.2-api (maven) versions >= 2.7, < 2.25.4 is affected.
- Is there a fix for CVE-2026-34479? Yes. CVE-2026-34479 is fixed in 2.25.4. Upgrade to this version or later.
- Is CVE-2026-34479 exploitable, and should I be worried? Whether CVE-2026-34479 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-2026-34479 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-2026-34479? Upgrade
org.apache.logging.log4j:log4j-1.2-apito 2.25.4 or later.