Vulnerability in Apache Log4cxx
CVE-2026-40023
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.
EPSS: 0.005 (39.9th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
Affected products
- Apache Log4cxx
- Apache Software Foundation Log4cxx — versions 0
- Apache Software Foundation Log4cxx (Brew) — versions 0
- Apache Software Foundation Log4cxx (Conan) — versions 0
Weakness classification (CWE)
References
- security@apache.org (Patch, patch, Issue Tracking)
- security@apache.org (vendor-advisory, Vendor Advisory)
- security@apache.org (Product, vendor-advisory)
- security@apache.org (related, Product)
- security@apache.org (vendor-advisory, Mailing List, Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Mailing List, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-40023?
- CVE-2026-40023 is a medium-severity vulnerability in Apache Log4cxx, classified under Improper Encoding or Escaping of Output. CVSS score: 5.3/10. Published 2026-04-10.
- How severe is CVE-2026-40023?
- Medium severity. CVSS v3 base score is 5.3 out of 10.