Apache Log4j 1XmlLayout XML Escape Flaw Fixed in 2.25.4
CVE-2026-34479 Published on April 10, 2026

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 Log4j1XmlLayout directly in a Log4j Core 2 configuration file. * Those using the Log4j 1 configuration compatibility layer with org.apache.log4j.xml.XMLLayout specified 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 https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html , and specifically the section on eliminating reliance on the bridge.

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Timeline

2026-02-16

Vulnerability reported by Ap4sh and ethicxz

2026-03-15

Independent report received from jabaltarik1 27 days later.

2026-03-24

Fix shared publicly by Piotr P. Karwasz as pull request #4078 9 days later.

2026-03-25

Fix verified by reporter 1 day later.

2026-03-28

Log4j 2.25.4 released 3 days later.

Weakness Type

What is an Output Sanitization Vulnerability?

The software prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

CVE-2026-34479 has been classified to as an Output Sanitization vulnerability or weakness.

Products Associated with CVE-2026-34479

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Apache Log4j 1 2 Api

Affected Versions

Apache Software Foundation Apache Log4j 1 to Log4j 2 bridge:

Version 2.7 and below 2.25.4 is affected.
Version 3.0.0-alpha1, <= 3.0.0-beta2 is affected.

Exploit Probability

EPSS

0.12%

Percentile

30.81%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.