CVE-2026-34479
Apache Log4j 1 to Log4j 2 bridge: Silent log event loss in Log4j1XmlLayout due to unescaped XML 1.0 forbidden characters
- State
- Published
- Severity
- 6.9 Medium
- Published
- Apr 10, 2026
- Updated
- Apr 10, 2026
- Reserved
- Mar 28, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 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 | apache |
Weaknesses
- CWE-116 Improper Encoding or Escaping of Output
Affected Products
| Product | Versions |
|---|---|
Apache Log4j 1 to Log4j 2 bridgeApache Software Foundation |
|
References
- https://github.com/apache/logging-log4j2/pull/4078patch
- https://logging.apache.org/security.html#CVE-2026-34479vendor-advisory
- https://logging.apache.org/cyclonedx/vdr.xmlvendor-advisory
- https://logging.apache.org/log4j/2.x/migrate-from-log4j1.htmlrelated
- https://lists.apache.org/thread/gd0hp6mj17rn3kj279vgy4p7kd4zz5onvendor-advisory
- http://www.openwall.com/lists/oss-security/2026/04/10/8
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