What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-10 · updated 2026-10-26
• Weakness: CWE-116

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.

Affected products

• Apache Log4j: from 2.0 before 2.25.4; 3.0.0-alpha1, 3.0.0-alpha1_rc1, 3.0.0-alpha1_rc2, 3.0.0-beta1, 3.0.0-beta2, 3.0.0-beta3 — fixed in 2.25.4
• Maven Org.Apache.Logging.Log4j:Log4j-Core: from 2.0-alpha1 before 2.25.4; from 3.0.0-alpha1 through 3.0.0-beta3; 2.0, 2.0-alpha1, 2.0-alpha2, 2.0-beta1, 2.0-beta2, 2.0-beta3 (+68) — fixed in 2.25.4

How to fix

Upgrade to a fixed version: Apache Log4j 2.25.4; Maven Org.Apache.Logging.Log4j:Log4j-Core 2.25.4. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4j2/pull/4077
• https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb
• https://logging.apache.org/cyclonedx/vdr.xml
• https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout
• https://logging.apache.org/security.html#CVE-2026-34480
• http://www.openwall.com/lists/oss-security/2026/04/10/9

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