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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 · CISA-ADP (v2.0.3)
Decision recorded 2026-04-10 17:45 UTC
CWE
  • CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
Impacted products
Vendor Product Version CPE status
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:*:*:*:*:*:*:*:*
Create a notification for this product.
Credits
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) jabaltarik1 (independently)
Show details on NVD website
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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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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.