Apache Log4j 1 to Log4j 2 bridge: Silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters

Summary

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.

Timeline

1. Apr 10, 2026, 3:41 PM UTCCVE published
2. Apr 10, 2026, 5:45 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Apr 10, 2026, 5:47 PM UTCRecord last updated

Affected versions

• org.apache.logging.log4j:log4j-1.2-api

  From 2.7 · fixed in 2.25.4; From 3.0.0-alpha1 · through 3.0.0-beta2

• org.apache.logging.log4j:log4j-coreMaven

  From 2.7 · fixed in 2.25.4

• org.apache.logging.log4j:log4j-coreMaven

  From 3.0.0-beta1 · through 3.0.0-beta2

References 5