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        "providerMetadata": {
          "dateUpdated": "2026-04-10T16:18:21.980Z",
          "orgId": "af854a3a-2127-422b-91ae-364da2661108",
          "shortName": "CVE"
        },
        "references": [
          {
            "url": "http://www.openwall.com/lists/oss-security/2026/04/10/11"
          }
        ],
        "title": "CVE Program Container"
      },
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-40021",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-04-10T17:32:06.208358Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
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        ],
        "providerMetadata": {
          "dateUpdated": "2026-04-10T17:35:01.228Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "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": "\u003cp\u003eApache Log4net\u0027s \u003ccode\u003e\u003ca target=\"_blank\" rel=\"nofollow\" href=\"https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list\"\u003eXmlLayout\u003c/a\u003e\u003c/code\u003e and \u003ccode\u003e\u003ca target=\"_blank\" rel=\"nofollow\" href=\"https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list\"\u003eXmlLayoutSchemaLog4J\u003c/a\u003e\u003c/code\u003e, in versions before 3.3.0, fail to sanitize characters forbidden by the \u003ca target=\"_blank\" rel=\"nofollow\" href=\"https://www.w3.org/TR/xml/#charsets\"\u003eXML 1.0 specification\u003c/a\u003e 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.\u003c/p\u003e\u003cp\u003eAn attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity.\u003c/p\u003e\u003cp\u003eUsers are advised to upgrade to Apache Log4net \u003ccode\u003e3.3.0\u003c/code\u003e, which fixes this issue.\u003c/p\u003e"
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          ],
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        }
      ],
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            "Automatable": "NOT_DEFINED",
            "Recovery": "NOT_DEFINED",
            "Safety": "NOT_DEFINED",
            "attackComplexity": "HIGH",
            "attackRequirements": "NONE",
            "attackVector": "NETWORK",
            "baseScore": 6.3,
            "baseSeverity": "MEDIUM",
            "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",
            "version": "4.0",
            "vulnAvailabilityImpact": "NONE",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "NONE",
            "vulnerabilityResponseEffort": "NOT_DEFINED"
          },
          "format": "CVSS",
          "scenarios": [
            {
              "lang": "en",
              "value": "GENERAL"
            }
          ]
        },
        {
          "other": {
            "content": {
              "text": "moderate"
            },
            "type": "Textual description of severity"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-116",
              "description": "CWE-116 Improper Encoding or Escaping of Output",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-04-10T15:44:17.451Z",
        "orgId": "f0158376-9dc2-43b6-827c-5f631a4d8d09",
        "shortName": "apache"
      },
      "references": [
        {
          "tags": [
            "patch"
          ],
          "url": "https://github.com/apache/logging-log4net/pull/280"
        },
        {
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://logging.apache.org/security.html#CVE-2026-40021"
        },
        {
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://logging.apache.org/cyclonedx/vdr.xml"
        },
        {
          "tags": [
            "related"
          ],
          "url": "https://logging.apache.org/log4net/manual/configuration/layouts.html"
        },
        {
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7"
        }
      ],
      "source": {
        "discovery": "EXTERNAL"
      },
      "timeline": [
        {
          "lang": "en",
          "time": "2026-02-10T23:34:00.000Z",
          "value": "Vulnerability reported by f00dat"
        },
        {
          "lang": "en",
          "time": "2026-02-17T08:03:00.000Z",
          "value": "Fix shared publicly by Jan Friedrich as pull request #280"
        },
        {
          "lang": "en",
          "time": "2026-02-20T13:12:00.000Z",
          "value": "Log4net 3.3.0 released"
        }
      ],
      "title": "Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters",
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    "assignerShortName": "apache",
    "cveId": "CVE-2026-40021",
    "datePublished": "2026-04-10T15:44:17.451Z",
    "dateReserved": "2026-04-08T10:03:43.840Z",
    "dateUpdated": "2026-04-10T17:35:01.228Z",
    "state": "PUBLISHED"
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  "dataVersion": "5.2"
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