CVE-2026-40021: Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters

[Piotr Karwasz <pk...@apache.org>]

Severity: moderate 

Affected versions:

- Apache Log4net (log4net) before 3.3.0

Description:

Apache Log4net's  XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list  and  XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the  XML 1.0 specification https://www.w3.org/TR/xml/#charsets  in MDC property keys and values, as well as the identity field that may carry attacker-influenced data. This causes an exception during serialization and the silent loss of the affected log event.

An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity.

Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.

Credit:

f00dat (finder)

References:

https://github.com/apache/logging-log4net/pull/280
https://logging.apache.org/security.html#CVE-2026-40021
https://logging.apache.org/cyclonedx/vdr.xml
https://logging.apache.org/log4net/manual/configuration/layouts.html
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-40021

Timeline:

2026-02-10: Vulnerability reported by f00dat
2026-02-17: Fix shared publicly by Jan Friedrich as pull request #280
2026-02-20: Log4net 3.3.0 released