CVE-2026-34479 (GCVE-0-2026-34479)

Vulnerability from cvelistv5 – Published: 2026-04-10 15:41 – Updated: 2026-04-10 17:47
VLAI
Title
Apache Log4j 1 to Log4j 2 bridge: Silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters
Summary
The Log4j1XmlLayout from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML 1.0 standard, producing malformed XML output. Conforming XML parsers are required to reject documents containing such characters with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records. Two groups of users are affected: * Those using Log4j1XmlLayout directly in a Log4j Core 2 configuration file. * Those using the Log4j 1 configuration compatibility layer with org.apache.log4j.xml.XMLLayout specified as the layout class. Users are advised to upgrade to Apache Log4j 1-to-Log4j 2 bridge version 2.25.4, which corrects this issue. Note: The Apache Log4j 1-to-Log4j 2 bridge is deprecated and will not be present in Log4j 3. Users are encouraged to consult the Log4j 1 to Log4j 2 migration guide https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html , and specifically the section on eliminating reliance on the bridge.
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 (v2.0.3)
CWE
  • CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
Impacted products
Vendor Product Version
Apache Software Foundation Apache Log4j 1 to Log4j 2 bridge Affected: 2.7 , < 2.25.4 (maven)
Affected: 3.0.0-alpha1 , ≤ 3.0.0-beta2 (maven)
    cpe:2.3:a:apache:log4j_1_2_api:*:*:*:*:*:*:*:*
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Credits
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) jabaltarik1 (independently)
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Karwasz as pull request #4078\"}, {\"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\"}], \"references\": [{\"url\": \"https://github.com/apache/logging-log4j2/pull/4078\", \"tags\": [\"patch\"]}, {\"url\": \"https://logging.apache.org/security.html#CVE-2026-34479\", \"tags\": [\"vendor-advisory\"]}, {\"url\": \"https://logging.apache.org/cyclonedx/vdr.xml\", \"tags\": [\"vendor-advisory\"]}, {\"url\": \"https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html\", \"tags\": [\"related\"]}, {\"url\": \"https://lists.apache.org/thread/gd0hp6mj17rn3kj279vgy4p7kd4zz5on\", \"tags\": [\"vendor-advisory\"]}], \"x_generator\": {\"engine\": \"Vulnogram 0.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"The Log4j1XmlLayout from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML 1.0 standard, producing malformed XML output. Conforming XML parsers are required to reject documents containing such characters with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records.\\n\\nTwo groups of users are affected:\\n\\n  *  Those using Log4j1XmlLayout directly in a Log4j Core 2 configuration file.\\n  *  Those using the Log4j 1 configuration compatibility layer with org.apache.log4j.xml.XMLLayout specified as the layout class.\\n\\n\\nUsers are advised to upgrade to Apache Log4j 1-to-Log4j 2 bridge version 2.25.4, which corrects this issue.\\n\\nNote: The Apache Log4j 1-to-Log4j 2 bridge is deprecated and will not be present in Log4j 3. Users are encouraged to consult the  Log4j 1 to Log4j 2 migration guide https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html , and specifically the section on eliminating reliance on the bridge.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eThe \u003ccode\u003eLog4j1XmlLayout\u003c/code\u003e from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML 1.0 standard, producing malformed XML output. Conforming XML parsers are required to reject documents containing such characters with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records.\u003c/p\u003e\u003cp\u003eTwo groups of users are affected:\u003c/p\u003e\u003cul\u003e\u003cli\u003eThose using \u003ccode\u003eLog4j1XmlLayout\u003c/code\u003e directly in a Log4j Core 2 configuration file.\u003c/li\u003e\u003cli\u003eThose using the Log4j 1 configuration compatibility layer with \u003ccode\u003eorg.apache.log4j.xml.XMLLayout\u003c/code\u003e specified as the layout class.\u003c/li\u003e\u003c/ul\u003e\u003cp\u003eUsers are advised to upgrade to Apache Log4j 1-to-Log4j 2 bridge version 2.25.4, which corrects this issue.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eNote:\u003c/strong\u003e The Apache Log4j 1-to-Log4j 2 bridge is deprecated and will not be present in Log4j 3. Users are encouraged to consult the \u003ca target=\\\"_blank\\\" rel=\\\"nofollow\\\" href=\\\"https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html\\\"\u003eLog4j 1 to Log4j 2 migration guide\u003c/a\u003e, and specifically the section on eliminating reliance on the bridge.\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\": \"2026-04-10T15:41:07.888Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-34479\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-04-10T17:47:34.402Z\", \"dateReserved\": \"2026-03-28T14:06:31.965Z\", \"assignerOrgId\": \"f0158376-9dc2-43b6-827c-5f631a4d8d09\", \"datePublished\": \"2026-04-10T15:41:07.888Z\", \"assignerShortName\": \"apache\"}",
      "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.