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

Medium 5.3 Published 6 Oct 2026 Updated 7 Oct 2026

Attackers can suppress Windows Event Log records by sending oversized messages, causing silent logging failures in applications using Apache log4net.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-105243?

CVE-2026-105243 is a medium-severity denial of service vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-778

Which versions are affected by CVE-2026-105243?

It affects 1 product.

Products include Apache Log4net.

How do I fix CVE-2026-105243?

No fix has been published for CVE-2026-105243 yet. Limit exposure of the affected software until one is available.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-105243 being exploited?

There is no known exploitation of CVE-2026-105243 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-105243, GHSA-j8mm-w573-8v6c, UBUNTU-CVE-2026-105243.