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CVE-2026-40021 — Apache Log4net: Silent log event loss in log4net...

MEDIUM

Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters

Published April 10, 2026Updated May 5, 2026Source: osv

Summary

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.

Advisory Timeline

  • First published April 10, 2026 (185 days ago).
  • Last revised May 5, 2026 — 25 days after initial publication.

Remediation

Upgrade to the fixed version using your package manager.

NuGet
Update log4net to 3.3.0 or later
dotnet add package log4net --version 3.3.0

After upgrading, run your dependency scanner again to confirm the vulnerability is resolved.

Affected Packages (1)

Affects 1 ecosystem: nuget (1).

PackageEcosystemAffectedFixed In
log4net
nuget
1.2.10, 1.2.11, 2.0.0, 2.0.1 (+23 more)3.3.0

Vulnerability Classification

Common Weakness Enumeration (CWE) identifiers for this vulnerability type.

  • CWE-116
    Improper Encoding or EscapingMITRE

CVSS Score Breakdown

What the CVSS (Common Vulnerability Scoring System) 5.3 score means for each attack dimension.

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

References

Frequently Asked Questions

What is CVE-2026-40021?
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters This vulnerability has been assigned a severity rating of MEDIUM (CVSS score: 5.3/10).
How do I check if my project is affected by CVE-2026-40021?
CVE-2026-40021 affects log4net. Use GeekWala's free vulnerability scanner to check your dependencies against CVE-2026-40021 and 200,000+ other known vulnerabilities.

Severity & Exploitability

CVSS Score
5.3

Exploitation requires specific conditions or has limited impact. Remediate within weeks.

Also Known As

GHSA-4f7c-pmjv-c25w

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