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
CWE
  • CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
Impacted products
Vendor Product Version
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:*:*:*:*:*:*:*:*
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Credits
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) jabaltarik1 (independently)
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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.\", \"supportingMedia\": [{\"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\", \"base64\": false}]}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-116\", \"description\": \"CWE-116 Improper Encoding or Escaping of Output\"}]}], \"providerMetadata\": {\"orgId\": \"f0158376-9dc2-43b6-827c-5f631a4d8d09\", \"shortName\": \"apache\", \"dateUpdated\": \"2026-04-10T15:42:03.843Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-34480\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-04-10T17:45:07.434Z\", \"dateReserved\": \"2026-03-28T15:29:27.095Z\", \"assignerOrgId\": \"f0158376-9dc2-43b6-827c-5f631a4d8d09\", \"datePublished\": \"2026-04-10T15:42:03.843Z\", \"assignerShortName\": \"apache\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}


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Tags
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.