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, and specifically the section on eliminating reliance on the bridge.