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: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-10 · updated 2026-10-26
Weakness: CWE-116

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.

Affected products

Apache Log4j: from 2.7 before 2.25.4; 3.0.0-alpha1, 3.0.0-alpha1_rc1, 3.0.0-alpha1_rc2, 3.0.0-beta1, 3.0.0-beta2 — fixed in 2.25.4
Maven Org.Apache.Logging.Log4j:Log4j-Core: from 2.7 before 2.25.4; from 3.0.0-beta1 through 3.0.0-beta2; 2.10.0, 2.11.0, 2.11.1, 2.11.2, 2.12.0, 2.12.1 (+39) — fixed in 2.25.4

How to fix

Upgrade to a fixed version: Apache Log4j 2.25.4; Maven Org.Apache.Logging.Log4j:Log4j-Core 2.25.4. Check the vendor advisory for mitigations.

References

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