{"id":"CVE-2026-40021","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-40021","generatedAt":"2026-10-28T00:17:07.653Z","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 3.3.0, fail to…","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.","published":"2026-04-10T16:16:32.000Z","lastModified":"2026-10-26T21:10:00.000Z","status":"Analyzed","sourceIdentifier":"security@apache.org","cvss":{"version":"4.0","score":6.3,"severity":"MEDIUM","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/M"},"cwe":["CWE-116"],"affected":["apache/log4net"],"epss":{"score":0.00772,"percentile":0.55426,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"low","text":"Lower severity and a low EPSS score (0.77%). Track it; it rarely justifies an emergency change on its own."},"changes":[],"references":[{"url":"https://github.com/apache/logging-log4net/pull/280","tags":["Issue Tracking"],"source":"security@apache.org"},{"url":"https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7","tags":["Mailing List","Vendor Advisory"],"source":"security@apache.org"},{"url":"https://logging.apache.org/cyclonedx/vdr.xml","tags":["Product"],"source":"security@apache.org"},{"url":"https://logging.apache.org/log4net/manual/configuration/layouts.html","tags":["Product"],"source":"security@apache.org"},{"url":"https://logging.apache.org/security.html#CVE-2026-40021","tags":["Vendor Advisory"],"source":"security@apache.org"},{"url":"http://www.openwall.com/lists/oss-security/2026/04/10/11","tags":["Mailing List","Third Party Advisory"],"source":"af854a3a-2127-422b-91ae-364da2661108"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-40021","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}