Home/CVE/Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLa
CVE

CVE-2026-40021

Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLa

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.

MEDIUM · CVSS 5.3 EPSS 0.00285
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
Sigma rules0 YARA rules0
⬡

Weakness Classification

CWE-116Improper Encoding or Escaping of Output
▤

Affected Products & Versions

1
apache log4net< 3.3.0
▤

Affected Packages

1
Language-ecosystem packages (from OSV) tied to this CVE, with the version that fixes it - the dependency-level detail NVD doesn’t carry.
NuGet log4net MODERATE fixed in 3.3.0
▣

Scoring & Timeline

5.3
MEDIUM · CVSS v3.1 · security@apache.org
View on NVD
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
Published to NVD10 Apr 2026 · 04:16 PM
CVSS VectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
SSVC triage · cisa-vulnrichment
Exploitation
none
Automatable
no
Technical impact
partial
SSVC asks the questions that actually drive patch urgency: is it being exploited, can attacks be automated, and how total is the impact.
🔗

References & Sources

6
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://github.com/apache/logging-log4net/pull/280Issue Tracking
https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7Mailing ListVendor Advisory
https://logging.apache.org/cyclonedx/vdr.xmlProduct
https://logging.apache.org/log4net/manual/configuration/layouts.htmlProduct
https://logging.apache.org/security.html#CVE-2026-40021Vendor Advisory
http://www.openwall.com/lists/oss-security/2026/04/10/11Mailing ListThird Party Advisory
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