Silent log event loss in Apache Log4j 1-to-Log4j 2 bridge Log4j1XmlLayout
CVE-2026-34479 is a moderate-severity vulnerability in the Apache Log4j 1-to-Log4j 2 bridge component, specifically in Log4j1XmlLayout. The layout fails to escape characters forbidden by the XML 1.0 standard, resulting in malformed XML output. Affected software is org.apache.logging.log4j:log4j-1.2-api in versions 2.7 through before 2.25.4, and 3.0.0-alpha1 through 3.0.0-beta2. Two affected usage patterns are identified: direct use of Log4j1XmlLayout in a Log4j Core 2 configuration, and use of the Log4j 1 compatibility layer with org.apache.log4j.xml.XMLLayout configured as the layout class. Because conforming XML parsers must reject documents containing forbidden XML 1.0 characters with a fatal error, malformed log output can break downstream parsing and processing.
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