CVE-2026-105243: Apache log4net: Oversize EventLogAppender record silently discarded

[Jan Friedrich <freeandnil () apache org>]

Severity: moderate 
    CVSS 3.1: 5.3 (medium) CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected versions:

- Apache log4net 1.2.9 before 3.5.0
- Apache log4net 02e1e115435888485f2e28b414d267e39e799e07 before 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed

Description:

Insufficient Logging vulnerability in the EventLogAppender of Apache log4net.

Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source 
names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message 
could suppress the whole record by making it long enough. Only applications on Windows that use EventLogAppender are 
affected.

This issue affects Apache log4net: from 1.2.9 before 3.5.0.

Users are recommended to upgrade to version 3.5.0, which fixes the issue.

Credit:

The Apache Software Foundation (finder)
Claude Security (tool)
Jan Friedrich (remediation developer)

References:

https://github.com/apache/logging-log4net/pull/315
https://github.com/apache/logging-log4net/commit/28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-105243