CVE-2026-34480 Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters

What is CVE-2026-34480?

CVE-2026-34480 is a medium-severity vulnerability in Apache Log4j Core, published on 10 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-116

Which versions are affected by CVE-2026-34480?

4 open source packages ship an affected version, including org.apache.logging.log4j:log4j-core. It affects 1 product.

Products include Apache Log4j.

How do I fix CVE-2026-34480?

Upgrade org.apache.logging.log4j:log4j-core to 2.25.4 or later.

Is CVE-2026-34480 being exploited?

There is no known exploitation of CVE-2026-34480 in the wild and no public exploit code.

Other vulnerabilities in Apache Log4j Core

• CVE-2021-44228 Apache Log4j2 Remote Code Execution Exploited Critical 10.0
• CVE-2021-45046 Apache Log4j2 Deserialization of Untrusted Data Exploited Critical 9.0
• CVE-2022-23305 SQL injection in JDBC Appender in Apache Log4j V1 Critical 9.8
• CVE-2017-5645 Deserialization of Untrusted Data in Log4j Critical 9.8
• CVE-2022-23302 Deserialization of untrusted data in JMSSink in Apache Log4j 1.x High 8.8

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as CLEANSTART-2026-BH90179, CLEANSTART-2026-GM49347, CLEANSTART-2026-JA22493, CLEANSTART-2026-JH04977, CLEANSTART-2026-OV82681, CLEANSTART-2026-YQ06926 and 102 more.