Vulnerability 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.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Apache | Log4Net | < 3.3.0 |
Related Weaknesses (CWE)
References
- https://github.com/apache/logging-log4net/pull/280Issue Tracking
- https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7Mailing ListVendor Advisory
- https://logging.apache.org/cyclonedx/vdr.xmlProduct
- https://logging.apache.org/log4net/manual/configuration/layouts.htmlProduct
- https://logging.apache.org/security.html#CVE-2026-40021Vendor Advisory
- http://www.openwall.com/lists/oss-security/2026/04/10/11Mailing ListThird Party Advisory
FAQ
What is CVE-2026-40021?
CVE-2026-40021 is a vulnerability with a CVSS score of 5.3 (MEDIUM). 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....
How severe is CVE-2026-40021?
CVE-2026-40021 has been rated MEDIUM with a CVSS base score of 5.3/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-40021?
Check the references section above for vendor advisories and patch information. Affected products include: Apache Log4Net.