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.
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
Affected packages
Versions sourced from the GitHub Security Advisory.
| Package | Affected versions | Patched versions |
|---|---|---|
log4netNuGet | < 3.3.0 | 3.3.0 |
Affected products
2Patches
Vulnerability mechanics
References
8- www.openwall.com/lists/oss-security/2026/04/10/11nvdMailing ListThird Party AdvisoryWEB
- github.com/advisories/GHSA-4f7c-pmjv-c25wghsaADVISORY
- lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7nvdMailing ListVendor AdvisoryWEB
- logging.apache.org/security.htmlnvdVendor AdvisoryWEB
- nvd.nist.gov/vuln/detail/CVE-2026-40021ghsaADVISORY
- github.com/apache/logging-log4net/pull/280nvdIssue TrackingWEB
- logging.apache.org/cyclonedx/vdr.xmlnvdProductWEB
- logging.apache.org/log4net/manual/configuration/layouts.htmlnvdProductWEB
News mentions
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