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
