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

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Summary

CVECVE-2026-34479
StatePUBLISHED
Assignerapache
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-10 16:16:31 UTC
Updated2026-04-10 17:17:02 UTC
DescriptionThe 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.

Risk And Classification

Primary CVSS: v4.0 6.9 MEDIUM from [email protected]

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Problem Types: CWE-116 | CWE-116 CWE-116 Improper Encoding or Escaping of Output

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N/E:X/C...
4.0CNACVSS6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
None
Availability
None
Sub Conf.
None
Sub Integrity
Low
Sub Availability
None

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Apache Software Foundation Apache Log4j 1 To Log4j 2 Bridge affected 2.7 2.25.4 maven Not specified
CNA Apache Software Foundation Apache Log4j 1 To Log4j 2 Bridge affected 3.0.0-alpha1 3.0.0-beta2 maven Not specified

References

ReferenceSourceLinkTags
www.openwall.com/lists/oss-security/2026/04/10/8 af854a3a-2127-422b-91ae-364da2661108 www.openwall.com
github.com/apache/logging-log4j2/pull/4078 [email protected] github.com
logging.apache.org/security.html [email protected] logging.apache.org
logging.apache.org/log4j/2.x/migrate-from-log4j1.html [email protected] logging.apache.org
logging.apache.org/cyclonedx/vdr.xml [email protected] logging.apache.org
lists.apache.org/thread/gd0hp6mj17rn3kj279vgy4p7kd4zz5on [email protected] lists.apache.org
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) (en)

CNA: jabaltarik1 (independently) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-02-16T17:11:00.000ZVulnerability reported by Ap4sh and ethicxz
CNA2026-03-15T05:39:00.000ZIndependent report received from jabaltarik1
CNA2026-03-24T18:56:00.000ZFix shared publicly by Piotr P. Karwasz as pull request #4078
CNA2026-03-25T09:46:00.000ZFix verified by reporter
CNA2026-03-28T11:19:00.000ZLog4j 2.25.4 released
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