What’s Vulnerable › Vulnerabilities

CVE-2026-40023: Apache Log4cxx, Apache Log4cxx (Conan), Apache Log4cxx (Brew): Silent log event loss in XMLLayout due to unescaped XML 1.0 forbidden characters

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% 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 Log4cxx's XMLLayout https://logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html , in versions before 1.7.0, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in log messages, NDC, and MDC property keys and values, producing invalid XML output. Conforming XML parsers must reject such documents with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records. An attacker who can influence logged data can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4cxx 1.7.0, which fixes this issue.

Affected products

• Apache Software Foundation Apache Log4cxx: before 1.7.0 — fixed in 1.7.0

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache Log4cxx 1.7.0. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4cxx/pull/609
• https://lists.apache.org/thread/y15cv3zblg3dfwr5vy6ddbnl4zyrzr8b
• https://logging.apache.org/cyclonedx/vdr.xml
• https://logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html
• https://logging.apache.org/security.html#CVE-2026-40023
• http://www.openwall.com/lists/oss-security/2026/04/10/12

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