What’s Vulnerable › Vulnerabilities

CVE-2026-40021: Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-10 · updated 2026-10-26
• Weakness: CWE-116

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.

Affected products

• Apache Software Foundation Apache log4net: before 3.3.0 — fixed in 3.3.0

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache log4net 3.3.0. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4net/pull/280
• https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7
• https://logging.apache.org/cyclonedx/vdr.xml
• https://logging.apache.org/log4net/manual/configuration/layouts.html
• https://logging.apache.org/security.html#CVE-2026-40021
• http://www.openwall.com/lists/oss-security/2026/04/10/11

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