Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
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.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| .NET (NuGet) | log4net | < 3.3.0 | 3.3.0 |
References
- https://nvd.nist.gov/vuln/detail/CVE-2026-40021
- https://github.com/apache/logging-log4net/pull/280
- https://github.com/apache/logging-log4net
- 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
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N- Weakness (CWE)
CWE-116- Platform
- .NET (NuGet)
- Package
log4net- Disclosed
- 10.04.2026
- Updated
- 11.10.2026 11:23
Sources and licenses
OSV / GitHub Advisory Database - GHSA-4f7c-pmjv-c25w - CC BY 4.0
Vulnerability history: log4net
All published vulnerabilities of this package (.NET (NuGet)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 3
- First → latest
- 01.2021 04.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- Medium 2
Disclosures per year
- 2021: 1
- 2022: 1
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 1
-
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters this entry
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.