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CVE-2026-34480 (GCVE-0-2026-34480)

Vulnerability from cvelistv5 – Published: 2026-04-10 15:42 – Updated: 2026-04-10 17:45
VLAI
Title
Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters
Summary
Apache Log4j Core's XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in versions up to and including 2.25.3, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets producing invalid XML output whenever a log message or MDC value contains such characters. The impact depends on the StAX implementation in use: * JRE built-in StAX: Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records. * Alternative StAX implementations (e.g., Woodstox https://github.com/FasterXML/woodstox , a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j's internal status logger. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output.
Severity
6.9 (Medium)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
SSVC
Exploitation: none Automatable: yes Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-04-10 17:43 UTC
CWE
  • CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
Impacted products
Vendor Product Version CPE status
Apache Software Foundation Apache Log4j Core Affected: 2.0-alpha1 , < 2.25.4 (maven)
Affected: 3.0.0-alpha1 , ≤ 3.0.0-beta3 (maven)
    cpe:2.3:a:apache:log4j:*:*:*:*:*:*:*:*
Create a notification for this product.
Credits
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) jabaltarik1 (independently)
Show details on NVD website
To clipboard 

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                        "Automatable": "yes"
                      },
                      {
                        "Technical Impact": "partial"
                      }
                    ],
                    "role": "CISA Coordinator",
                    "timestamp": "2026-04-10T17:43:51.255638Z",
                    "version": "2.0.3"
                  },
                  "type": "ssvc"
                }
              }
            ],
            "providerMetadata": {
              "dateUpdated": "2026-04-10T17:45:00.601Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "collectionURL": "https://repo.maven.apache.org/maven2",
              "cpes": [
                "cpe:2.3:a:apache:log4j:*:*:*:*:*:*:*:*"
              ],
              "defaultStatus": "unaffected",
              "packageName": "org.apache.logging.log4j:log4j-core",
              "packageURL": "pkg:maven/org.apache.logging.log4j/log4j-core",
              "product": "Apache Log4j Core",
              "vendor": "Apache Software Foundation",
              "versions": [
                {
                  "lessThan": "2.25.4",
                  "status": "affected",
                  "version": "2.0-alpha1",
                  "versionType": "maven"
                },
                {
                  "lessThanOrEqual": "3.0.0-beta3",
                  "status": "affected",
                  "version": "3.0.0-alpha1",
                  "versionType": "maven"
                }
              ]
            }
          ],
          "credits": [
            {
              "lang": "en",
              "type": "finder",
              "value": "Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters)"
            },
            {
              "lang": "en",
              "type": "finder",
              "value": "jabaltarik1 (independently)"
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "supportingMedia": [
                {
                  "base64": false,
                  "type": "text/html",
                  "value": "\u003cp\u003eApache Log4j Core\u0027s \u003ccode\u003e\u003ca target=\"_blank\" rel=\"nofollow\" href=\"https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout\"\u003eXmlLayout\u003c/a\u003e\u003c/code\u003e, in versions up to and including 2.25.3, fails 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 producing invalid XML output whenever a log message or MDC value contains such characters.\u003c/p\u003e\u003cp\u003eThe impact depends on the StAX implementation in use:\u003c/p\u003e\u003cul\u003e\u003cli\u003e\u003cstrong\u003eJRE built-in StAX:\u003c/strong\u003e Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records.\u003c/li\u003e\u003cli\u003e\u003cstrong\u003eAlternative StAX implementations\u003c/strong\u003e (e.g., \u003ca target=\"_blank\" rel=\"nofollow\" href=\"https://github.com/FasterXML/woodstox\"\u003eWoodstox\u003c/a\u003e, a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j\u0027s internal status logger.\u003c/li\u003e\u003c/ul\u003e\u003cp\u003eUsers are advised to upgrade to Apache Log4j Core \u003ccode\u003e2.25.4\u003c/code\u003e, which corrects this issue by sanitizing forbidden characters before XML output.\u003c/p\u003e"
                }
              ],
              "value": "Apache Log4j Core\u0027s  XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in versions up to and including 2.25.3, fails to sanitize characters forbidden by the  XML 1.0 specification https://www.w3.org/TR/xml/#charsets  producing invalid XML output whenever a log message or MDC value contains such characters.\n\nThe impact depends on the StAX implementation in use:\n\n  *  JRE built-in StAX: Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records.\n  *  Alternative StAX implementations (e.g.,  Woodstox https://github.com/FasterXML/woodstox , a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j\u0027s internal status logger.\n\n\nUsers are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output."
            }
          ],
          "metrics": [
            {
              "cvssV4_0": {
                "Automatable": "NOT_DEFINED",
                "Recovery": "NOT_DEFINED",
                "Safety": "NOT_DEFINED",
                "attackComplexity": "LOW",
                "attackRequirements": "NONE",
                "attackVector": "NETWORK",
                "baseScore": 6.9,
                "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:L/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": "2026-04-10T15:42:03.843Z",
            "orgId": "f0158376-9dc2-43b6-827c-5f631a4d8d09",
            "shortName": "apache"
          },
          "references": [
            {
              "tags": [
                "patch"
              ],
              "url": "https://github.com/apache/logging-log4j2/pull/4077"
            },
            {
              "tags": [
                "vendor-advisory"
              ],
              "url": "https://logging.apache.org/security.html#CVE-2026-34480"
            },
            {
              "tags": [
                "vendor-advisory"
              ],
              "url": "https://logging.apache.org/cyclonedx/vdr.xml"
            },
            {
              "tags": [
                "related"
              ],
              "url": "https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout"
            },
            {
              "tags": [
                "vendor-advisory"
              ],
              "url": "https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb"
            }
          ],
          "source": {
            "discovery": "EXTERNAL"
          },
          "timeline": [
            {
              "lang": "en",
              "time": "2026-02-16T17:11:00.000Z",
              "value": "Vulnerability reported by Ap4sh and ethicxz"
            },
            {
              "lang": "en",
              "time": "2026-03-10T20:15:00.000Z",
              "value": "Candidate patch shared internally by Piotr P. Karwasz"
            },
            {
              "lang": "en",
              "time": "2026-03-15T05:39:00.000Z",
              "value": "Independent report received from jabaltarik1"
            },
            {
              "lang": "en",
              "time": "2026-03-24T18:52:00.000Z",
              "value": "Fix shared publicly by Piotr P. Karwasz as pull request #4077"
            },
            {
              "lang": "en",
              "time": "2026-03-25T09:46:00.000Z",
              "value": "Fix verified by reporter"
            },
            {
              "lang": "en",
              "time": "2026-03-28T11:19:00.000Z",
              "value": "Log4j 2.25.4 released"
            }
          ],
          "title": "Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters",
          "x_generator": {
            "engine": "Vulnogram 0.2.0"
          }
        }
      },
      "cveMetadata": {
        "assignerOrgId": "f0158376-9dc2-43b6-827c-5f631a4d8d09",
        "assignerShortName": "apache",
        "cveId": "CVE-2026-34480",
        "datePublished": "2026-04-10T15:42:03.843Z",
        "dateReserved": "2026-03-28T15:29:27.095Z",
        "dateUpdated": "2026-04-10T17:45:07.434Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}



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Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.