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Medium, CVSS 6.9amended

Apache Log4j 1 to Log4j 2 bridge: silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters

CVE-2026-34479GHSA-h383-gmxw-35v2

What to do

Partial vendor fix. Follow the primary source's mitigation guidance and check back for a fix.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · 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 · score source
EPSS
  • CVE-2026-34479: 0.53% estimated exploitation activity in the next 30 days; percentile 45.18 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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, and specifically the section on eliminating reliance…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from osv record GHSA-h383-gmxw-35v2

  2. Revision 2 · amended ·

    Amended from ghsa record GHSA-h383-gmxw-35v2

  3. Revision 1 · created ·

    Published from ghsa record GHSA-h383-gmxw-35v2

Agent brief

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# Patch Next advisory brief: CVE-2026-34479

Advisory: Apache Log4j 1 to Log4j 2 bridge: silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters
IDs: CVE-2026-34479, GHSA-h383-gmxw-35v2
Record: https://patchnext.com/advisories/cve-2026-34479 (revision 3, source checked 27 Oct 2026, 06:25 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: org.apache.logging.log4j:log4j-core 2.25.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.apache.logging.log4j:log4j-core: affected >= 2.7, < 2.25.4; >= 3.0.0-beta1, <= 3.0.0-beta2; fixed in 2.25.4

## Suggested inspection (read-only)
- Maven org.apache.logging.log4j:log4j-core: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.apache.logging.log4j:log4j-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-h383-gmxw-35v2 (GitHub-reviewed advisory GHSA-h383-gmxw-35v2 lists affected ranges: Maven org.apache.logging.log4j:log4j-core >=2.7 =3.0.0-beta1 <=3.0.0-beta2)
- Verified: https://osv.dev/vulnerability/GHSA-h383-gmxw-35v2 (OSV record GHSA-h383-gmxw-35v2 lists affected ranges: Maven org.apache.logging.log4j:log4j-core >=2.7 =3.0.0-beta1 <=3.0.0-beta2)

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