VulnerabilityAnalyzed

CVE-2026-40021

MEDIUM 6.3EPSS 0.75%

Does this matter?

Lower severity and a low EPSS score (0.75%). Track it; it rarely justifies an emergency change on its own.

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.

CVSS 4.0: 6.3 MEDIUMCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
EPSS: 0.75% probability · 53th percentile
CISA KEV: Not listed
Weakness: CWE-116
Affected: apache/log4net
Source: security@apache.org

References

https://github.com/apache/logging-log4net/pull/280Issue Tracking
https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7Mailing List, Vendor Advisory
https://logging.apache.org/cyclonedx/vdr.xmlProduct
https://logging.apache.org/log4net/manual/configuration/layouts.htmlProduct
https://logging.apache.org/security.html#CVE-2026-40021Vendor Advisory
http://www.openwall.com/lists/oss-security/2026/04/10/11Mailing List, Third Party Advisory

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.