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.
Fix
Patch, vendor advisory and mitigation references on NVD 2
- https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 Mailing List, Vendor Advisory
- https://logging.apache.org/security.html#CVE-2026-40021 Vendor Advisory
Every line above is quoted from NVD, CISA, Microsoft, the vendor's own advisory or DISA; nothing is generated.
Change history 1
- First seenCVSS 6.3 · apacheNVD modified 2026-10-26 21:10
Affected (CPE criteria) 1
cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*
References 6
- https://github.com/apache/logging-log4net/pull/280 Issue Tracking
- https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 Mailing List, Vendor Advisory
- https://logging.apache.org/cyclonedx/vdr.xml Product
- https://logging.apache.org/log4net/manual/configuration/layouts.html Product
- https://logging.apache.org/security.html#CVE-2026-40021 Vendor Advisory
- http://www.openwall.com/lists/oss-security/2026/04/10/11 Mailing List, Third Party Advisory