CVE-2026-40021

low-risk

Published 2026-04-10

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.

Do I need to act?

0.25% chance of exploitation

EPSS score — low exploit probability

Not on CISA KEV list

No confirmed active exploitation reported to CISA

Patch status unknown

Check vendor advisories for fix availability and mitigation guidance

CVSS 5.3/10 Medium

NETWORK / LOW complexity

Affected Products (1)

Log4Net

Affected Vendors

Apache

References (6)

Issue Tracking https://github.com/apache/logging-log4net/pull/280

Mailing List https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7

Product https://logging.apache.org/cyclonedx/vdr.xml

Product https://logging.apache.org/log4net/manual/configuration/layouts.html

Vendor Advisory https://logging.apache.org/security.html#CVE-2026-40021

Mailing List http://www.openwall.com/lists/oss-security/2026/04/10/11

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low-risk

Severity 21/34 · High

Exploitability 1/34 · Minimal

Exposure 5/34 · Minimal