Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
Summary
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
Original description
The description as published by the source.
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.
Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-40021 log4net
- Ubuntu UBUNTU-CVE-2026-40021 log4net
References
- advisory github.com
- advisory lists.apache.org
- advisory logging.apache.org
- advisory logging.apache.org
- advisory nvd.nist.gov
- patch github.com
- web openwall.com
- web logging.apache.org
- web nuget.org