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.
History

Fri, 10 Apr 2026 18:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Fri, 10 Apr 2026 17:30:00 +0000

Type Values Removed Values Added
References

Fri, 10 Apr 2026 16:00:00 +0000

Type Values Removed Values Added
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.
Title Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
First Time appeared Apache
Apache log4net
Weaknesses CWE-116
CPEs cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*
Vendors & Products Apache
Apache log4net
References
Metrics cvssV4_0

{'score': 6.3, 'vector': 'CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N'}


cve-icon MITRE

Status: PUBLISHED

Assigner: apache

Published:

Updated: 2026-04-10T17:35:01.228Z

Reserved: 2026-04-08T10:03:43.840Z

Link: CVE-2026-40021

cve-icon Vulnrichment

Updated: 2026-04-10T16:18:21.980Z

cve-icon NVD

Status : Received

Published: 2026-04-10T16:16:32.420

Modified: 2026-04-10T17:17:12.030

Link: CVE-2026-40021

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

No data.