{"id":"CVE-2026-40021","revision":"72109a10786c40fd80ea5896e9fe43df","records":[{"revision":"72109a10786c40fd80ea5896e9fe43df","observedAt":"2026-10-27T10:09:56.341+00:00","statement":{"source":"nvd","id":"CVE-2026-40021","aliases":[],"title":"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.…","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.\n\nAn attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity.\n\nUsers are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.","state":0,"publishedAt":"2026-04-10T16:16:32.42+00:00","modifiedAt":"2026-10-26T21:10:00.17+00:00","affected":[],"metrics":[{"system":"CVSS 4.0 · security@apache.org · Secondary","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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","score":6.3,"severity":"MEDIUM"},{"system":"CVSS 3.1 · nvd@nist.gov · Primary","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","score":5.3,"severity":"MEDIUM"}],"references":["https://github.com/apache/logging-log4net/pull/280","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"],"sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-40021","license":"NIST NVD usage terms; underlying source attribution retained","enrichment":{"sourceStatus":"Analyzed","weaknesses":["CWE-116"],"productCriteria":["cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*"],"sourceDates":{"published":"2026-04-10T16:16:32.420","lastModified":"2026-10-26T21:10:00.170"},"relatedAdvisories":[],"note":"NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON."},"key":"nvd:CVE-2026-40021","identifiers":["CVE-2026-40021"]},"retained":false,"historicalAffected":[]}],"coverage":[]}