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CVE-2026-105243

Apache log4net: Oversize EventLogAppender record silently discarded

Apache Software Foundation Apache log4net
Open in ThreatlineRecord JSON
CVSS
5.3Medium
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.81%Chance of exploitation activity in the next 30 days, higher than 56% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in3.5.0Also 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffedCVE List
Published6 Oct 2026Updated 7 Oct 2026CVE List

What fixes it

  • 3.5.0Apache log4netCVE List
  • 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffedApache log4netCVE List

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

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.

Text from CVE List

Affected versions 2

  • Apache log4netaffected: 1.2.9 before 3.5.0CVE List
  • Apache log4netaffected: 02e1e115435888485f2e28b414d267e39e799e07 before 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffedCVE List

References 3