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: moderate 

Affected versions:

- Apache Log4cxx before 1.7.0
- Apache Log4cxx (Conan) (log4cxx) before 1.7.0
- Apache Log4cxx (Brew) (log4cxx) before 1.7.0

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.

Credit:

Olawale Titiloye (finder)

References:

https://github.com/apache/logging-log4cxx/pull/609
https://logging.apache.org/security.html#CVE-2026-40023
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/
https://www.cve.org/CVERecord?id=CVE-2026-40023

Timeline:

2026-03-15: Vulnerability reported by Olawale Titiloye
2026-03-16: Fix shared publicly by Stephen Webb as pull request #609
2026-04-04: Log4cxx 1.7.0 released