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Message-ID: <4f627e3a-dd1c-add4-35fc-94448d187f4b@apache.org>
Date: Fri, 10 Apr 2026 13:41:00 +0000
From: Piotr Karwasz <pkarwasz@...che.org>
To: oss-security@...ts.openwall.com
Subject: CVE-2026-34479: Apache Log4j 1 to Log4j 2 bridge: Silent log event
loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters
Severity: moderate
Affected versions:
- Apache Log4j 1 to Log4j 2 bridge (org.apache.logging.log4j:log4j-1.2-api) 2.7 before 2.25.4
- Apache Log4j 1 to Log4j 2 bridge (org.apache.logging.log4j:log4j-1.2-api) 3.0.0-alpha1 through 3.0.0-beta2
Description:
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.
Credit:
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) (finder)
jabaltarik1 (independently) (finder)
References:
https://github.com/apache/logging-log4j2/pull/4078
https://logging.apache.org/security.html#CVE-2026-34479
https://logging.apache.org/cyclonedx/vdr.xml
https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-34479
Timeline:
2026-02-16: Vulnerability reported by Ap4sh and ethicxz
2026-03-15: Independent report received from jabaltarik1
2026-03-24: Fix shared publicly by Piotr P. Karwasz as pull request #4078
2026-03-25: Fix verified by reporter
2026-03-28: Log4j 2.25.4 released
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