CVE-2026-34480

HighCVSS 7.5

Published: 10/04/2026(UTC)Updated: 26/10/2026(UTC)Translated: 26/10/2026(UTC)NVD NIST

Exploitation Probability (EPSS)

Elevated risk

1.05%

64th percentile - higher than 64% of all known CVEs

Summary

Apache Log4j Core XmlLayout in versions up to and including 2.25.3 fails to sanitize characters forbidden by XML 1.0, producing invalid XML when a log message or MDC value contains such characters. Impact depends on the StAX implementation: built-in StAX silently writes forbidden characters, while alternative implementations (e.g., Woodstox) throw an exception and the log event is not delivered.

Risk Assessment

This may cause downstream log-processing systems to drop affected records or lose log events, hindering monitoring and auditing.

Recommendation

Upgrade Apache Log4j Core to version 2.25.4, which sanitizes forbidden characters before XML output.

Other vulnerabilities in Apache Log4j Core

CVE-2026-34477Medium
The fix for CVE-2025-68161 in Apache Log4j Core was incomplete: it addressed hostname verification only via the log4j2.sslVerifyHostName system property, not via the verifyHostName attribute of the <Ssl> element. This attribute was silently ignored in all versions through 2.25.3, leaving TLS connections vulnerable to interception.

Original NVD description (English source)

Apache Log4j Core's XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in versions up to and including 2.25.3, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets producing invalid XML output whenever a log message or MDC value contains such characters. The impact depends on the StAX implementation in use: * JRE built-in StAX: Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records. * Alternative StAX implementations (e.g., Woodstox https://github.com/FasterXML/woodstox , a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j's internal status logger. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS