CVE-2026-40021

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.

How to fix CVE-2026-40021

To remediate CVE-2026-40021, upgrade the affected package to a fixed version below.

• —no fix listed
• —upgrade to 3.3.0 or later

Is CVE-2026-40021 being exploited?

Low — EPSS is 0.8%, meaning exploitation activity has not been observed at scale.

Affected packages (2)

• from 0
• from 0, < 3.3.0

CVSS scores

References (9)

• ADVISORYnvd.nist.gov/vuln/detail/CVE-2026-40021
• ADVISORYsecurity-tracker.debian.org/tracker/CVE-2026-40021
• PATCHgithub.com/apache/logging-log4net
• WEBwww.openwall.com/lists/oss-security/2026/04/10/11
• WEB