Apache Log4j Core up to 2.25.3/3.0.0-beta3 Configuration incorrect provision of specified functionality

Summaryinfo

A vulnerability marked as problematic has been reported in Apache Log4j Core up to 2.25.3/3.0.0-beta3. This vulnerability affects unknown code of the component Configuration Handler. The manipulation leads to incorrect provision of specified functionality. This vulnerability is listed as CVE-2026-34478. The attack may be initiated remotely. There is no available exploit. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Apache Log4j Core up to 2.25.3/3.0.0-beta3. This issue affects an unknown code of the component Configuration Handler. The manipulation with an unknown input leads to a incorrect provision of specified functionality vulnerability. Using CWE to declare the problem leads to CWE-684. The code does not function according to its published specifications, potentially leading to incorrect usage. Impacted is integrity. The summary by CVE is:

Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0 through 2.25.3, is vulnerable to log injection via CRLF sequences due to undocumented renames of security-relevant configuration attributes. Two distinct issues affect users of stream-based syslog services who configure Rfc5424Layout directly: * The newLineEscape attribute was silently renamed, causing newline escaping to stop working for users of TCP framing (RFC 6587), exposing them to CRLF injection in log output. * The useTlsMessageFormat attribute was silently renamed, causing users of TLS framing (RFC 5425) to be silently downgraded to unframed TCP (RFC 6587), without newline escaping. Users of the SyslogAppender are not affected, as its configuration attributes were not modified. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue.

The weakness was presented by Samuli Leinonen. It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-34478 since 03/28/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 04/10/2026).

Upgrading to version 2.25.4 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• Apache

Name

• Log4j Core

Version

• 2.25.0
• 2.25.1
• 2.25.2
• 2.25.3
• 3.0.0-beta1
• 3.0.0-beta2
• 3.0.0-beta3

License

• open-source

Website

• Vendor: https://www.apache.org/

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