Vulnerability · Published Apr 10, 2026 · Assigned by apache
CVE-2026-40023Apache Log4cxx, Apache Log4cxx (Conan), Apache Log4cxx (Brew): Silent log event loss in XMLLayout due to unescaped XML 1.0 forbidden characters
Medium 6.3
In plain English
A medium-severity security flaw in Apache Log4cxx from Apache Software Foundation. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 43% of all known vulnerabilities).
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | log4cxx | no fix listed | DEBIAN-CVE-2026-40023 |
| Debian:13 | log4cxx | no fix listed | DEBIAN-CVE-2026-40023 |
| Debian:14 | log4cxx | no fix listed | DEBIAN-CVE-2026-40023 |
| Ubuntu:22.04:LTS | log4cxx | no fix listed | UBUNTU-CVE-2026-40023 |
| Ubuntu:24.04:LTS | log4cxx | no fix listed | UBUNTU-CVE-2026-40023 |
| Ubuntu:25.10 | log4cxx | no fix listed | UBUNTU-CVE-2026-40023 |
| Ubuntu:26.04:LTS | log4cxx | no fix listed | UBUNTU-CVE-2026-40023 |
| Ubuntu:26.10 | log4cxx | no fix listed | UBUNTU-CVE-2026-40023 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Apache Software Foundation | Apache Log4cxx | 0 – before 1.7.0 |
| Apache Software Foundation | Apache Log4cxx (Conan) | 0 – before 1.7.0 |
| Apache Software Foundation | Apache Log4cxx (Brew) | 0 – before 1.7.0 |
Weakness: CWE-116 · Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
References
- https://github.com/apache/logging-log4cxx/pull/609 patch
- https://logging.apache.org/security.html#CVE-2026-40023 vendor-advisory
- https://logging.apache.org/cyclonedx/vdr.xml vendor-advisory
- https://logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html related
- https://lists.apache.org/thread/y15cv3zblg3dfwr5vy6ddbnl4zyrzr8b vendor-advisory
- http://www.openwall.com/lists/oss-security/2026/04/10/12