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.
Problem types
CWE-116 Improper Encoding or Escaping of Output
Product status
Any version before 3.3.0
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 |
Credits
f00dat
References
www.openwall.com/lists/oss-security/2026/04/10/11
github.com/apache/logging-log4net/pull/280
logging.apache.org/security.html
logging.apache.org/cyclonedx/vdr.xml
logging.apache.org/log4net/manual/configuration/layouts.html
lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7