Medium [CVE-2026-40021] Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and…
This medium-severity Apache Software Foundation advisory covers CVE-2026-40021 affecting Apache Log4net.
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Summary
Apache Log4net's XmlLayout and XmlLayoutSchemaLog4J, in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification 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.
- 3.3.0
Official advisory · high-confidence parse· fetched 2 months ago·verify at source
Mitigation
Upgrade to a fixed release: 3.3.0. That is the remediation for this advisory.
The vendor advisory may list additional interim mitigations or workarounds not captured here — review it before change work.
Official advisory · high-confidence parse· fetched 2 months ago·verify at source
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