Improper Encoding or Escaping of Output Affecting log4net package, versions *
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Start learning- Snyk IDSNYK-DEBIAN11-LOG4NET-16032309
- published14 Apr 2026
- disclosed10 Apr 2026
Introduced: 10 Apr 2026
NewCVE-2026-40021 (opens in a new tab)How to fix?
There is no fixed version for Debian:11 log4net.
NVD Description
Note: Versions mentioned in the description apply only to the upstream log4net package and not the log4net package as distributed by Debian.
See How to fix? for Debian:11 relevant fixed versions and status.
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.
References
- https://security-tracker.debian.org/tracker/CVE-2026-40021
- 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