Apache Log4j 1 to Log4j 2 Bridge up to 2.25.3/3.0.0-beta2 Log4j1XmlLayout escape output

Summaryinfo

A vulnerability was found in Apache Log4j 1 to Log4j 2 Bridge up to 2.25.3/3.0.0-beta2. It has been declared as problematic. This impacts an unknown function of the component Log4j1XmlLayout. The manipulation results in escape output. This vulnerability was named CVE-2026-34479. The attack may be performed from remote. There is no available exploit. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Apache Log4j 1 to Log4j 2 Bridge up to 2.25.3/3.0.0-beta2. It has been classified as problematic. Affected is some unknown processing of the component Log4j1XmlLayout. The manipulation with an unknown input leads to a escape output vulnerability. CWE is classifying the issue as CWE-116. The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved. This is going to have an impact on integrity. CVE summarizes:

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.

The weakness was presented by Ap4Sh. The advisory is available at github.com. This vulnerability is traded as CVE-2026-34479 since 03/28/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 04/10/2026).

Upgrading to version 2.25.4 eliminates this vulnerability.

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Productinfo

Vendor

• Apache

Name

• Log4j 1 to Log4j 2 Bridge

Version

• 2.25.0
• 2.25.1
• 2.25.2
• 2.25.3
• 3.0.0-beta1
• 3.0.0-beta2

License

• open-source

Website

• Vendor: https://www.apache.org/

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