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
| Link | Resource |
|---|---|
| https://github.com/apache/logging-log4net/pull/280 | Issue Tracking |
| https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 | Mailing List Vendor Advisory |
| https://logging.apache.org/cyclonedx/vdr.xml | Product |
| https://logging.apache.org/log4net/manual/configuration/layouts.html | Product |
| https://logging.apache.org/security.html#CVE-2026-40021 | Vendor Advisory |
| http://www.openwall.com/lists/oss-security/2026/04/10/11 | Mailing List Third Party Advisory |
Configurations
History
22 Apr 2026, 14:13
| Type | Values Removed | Values Added |
|---|---|---|
| References | () https://github.com/apache/logging-log4net/pull/280 - Issue Tracking | |
| References | () https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 - Mailing List, Vendor Advisory | |
| References | () https://logging.apache.org/cyclonedx/vdr.xml - Product | |
| References | () https://logging.apache.org/log4net/manual/configuration/layouts.html - Product | |
| References | () https://logging.apache.org/security.html#CVE-2026-40021 - Vendor Advisory | |
| References | () http://www.openwall.com/lists/oss-security/2026/04/10/11 - Mailing List, Third Party Advisory | |
| First Time |
Apache log4net
Apache |
|
| CVSS |
v2 : v3 : |
v2 : unknown
v3 : 5.3 |
| CPE | cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:* |
10 Apr 2026, 17:17
| Type | Values Removed | Values Added |
|---|---|---|
| References |
|
10 Apr 2026, 16:16
| Type | Values Removed | Values Added |
|---|---|---|
| New CVE |
Information
Published : 2026-04-10 16:16
Updated : 2026-04-22 14:13
NVD link : CVE-2026-40021
Mitre link : CVE-2026-40021
CVE.ORG link : CVE-2026-40021
JSON object : View
Products Affected
apache
- log4net
CWE
CWE-116
Improper Encoding or Escaping of Output