{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-40021", "assignerOrgId": "f0158376-9dc2-43b6-827c-5f631a4d8d09", "state": "PUBLISHED", "assignerShortName": "apache", "dateReserved": "2026-04-08T10:03:43.840Z", "datePublished": "2026-04-10T15:44:17.451Z", "dateUpdated": "2026-04-10T17:35:01.228Z"}, "containers": {"cna": {"affected": [{"collectionURL": "https://www.nuget.org/packages", "cpes": ["cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*"], "defaultStatus": "unaffected", "packageName": "log4net", "packageURL": "pkg:nuget/log4net", "product": "Apache Log4net", "vendor": "Apache Software Foundation", "versions": [{"lessThan": "3.3.0", "status": "affected", "version": "0", "versionType": "nuget"}]}], "credits": [{"lang": "en", "type": "finder", "value": "f00dat"}], "descriptions": [{"lang": "en", "supportingMedia": [{"base64": false, "type": "text/html", "value": "<p>Apache Log4net's <code><a target=\"_blank\" rel=\"nofollow\" href=\"https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list\">XmlLayout</a></code> and <code><a target=\"_blank\" rel=\"nofollow\" href=\"https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list\">XmlLayoutSchemaLog4J</a></code>, in versions before 3.3.0, fail to sanitize characters forbidden by the <a target=\"_blank\" rel=\"nofollow\" href=\"https://www.w3.org/TR/xml/#charsets\">XML 1.0 specification</a> 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.</p><p>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.</p><p>Users are advised to upgrade to Apache Log4net <code>3.3.0</code>, which fixes this issue.</p>"}], "value": "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. 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{"cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "id": "CVE-2026-40021", "lastModified": "2026-04-10T17:17:12.030", "metrics": {"cvssMetricV40": [{"cvssData": {"Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "Safety": "NOT_DEFINED", "attackComplexity": "HIGH", "attackRequirements": "NONE", "attackVector": "NETWORK", "availabilityRequirement": "NOT_DEFINED", "baseScore": 6.3, "baseSeverity": "MEDIUM", "confidentialityRequirement": "NOT_DEFINED", "exploitMaturity": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "privilegesRequired": "NONE", "providerUrgency": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "LOW", "userInteraction": "NONE", "valueDensity": "NOT_DEFINED", "vectorString": "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", "version": "4.0", "vulnAvailabilityImpact": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnerabilityResponseEffort": "NOT_DEFINED"}, "source": "security@apache.org", "type": "Secondary"}]}, "published": "2026-04-10T16:16:32.420", "references": [{"source": "security@apache.org", "url": "https://github.com/apache/logging-log4net/pull/280"}, {"source": "security@apache.org", "url": "https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7"}, {"source": "security@apache.org", "url": "https://logging.apache.org/cyclonedx/vdr.xml"}, {"source": "security@apache.org", "url": "https://logging.apache.org/log4net/manual/configuration/layouts.html"}, {"source": "security@apache.org", "url": "https://logging.apache.org/security.html#CVE-2026-40021"}, {"source": "af854a3a-2127-422b-91ae-364da2661108", "url": "http://www.openwall.com/lists/oss-security/2026/04/10/11"}], "sourceIdentifier": "security@apache.org", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-116"}], "source": "security@apache.org", "type": "Secondary"}]}

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