CVE-2026-40021

medium Apache
CVSS v3 Base Score
5.3
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS Score
0.2%
Exploitation probability in 30 days
Top 64% most likely to be exploited
Attack Characteristics
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
Low
Availability
None
Published: April 10, 2026 (33 days ago)
Last Modified: April 22, 2026
Vendor: Apache
Source: NVD

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.

CWE

CWE-116

Affected Products

apache log4net

References

🔗 github.com 🔗 lists.apache.org 🔗 logging.apache.org 🔗 logging.apache.org 🔗 logging.apache.org