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.

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.

Published 2026-04-10 · last modified 2026-06-17.

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.

EPSS exploit prediction

EPSS: 0.010 (60.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 61st percentile — 60.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.010 · change: +0.005

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)

CWE-116 — Improper Encoding or Escaping of Output

References

github.com/apache/logging-log4cxx/pull/609 (Patch, patch, Issue Tracking)
logging.apache.org/security.html (vendor-advisory, Vendor Advisory)
logging.apache.org/cyclonedx/vdr.xml (Product, vendor-advisory)
logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html (related, Product)
lists.apache.org/thread/y15cv3zblg3dfwr5vy6ddbnl4zyrzr8b (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.